[LoopVectorize] Don't vectorize loops when everything will be scalarized

[LoopVectorize] Don't vectorize loops when everything will be scalarized

This change prevents the loop vectorizer from vectorizing when all of the vector
types it generates will be scalarized. I've run into this problem on the PPC's QPX
vector ISA, which only holds floating-point vector types. The loop vectorizer
will, however, happily vectorize loops with purely integer computation. Here's
an example:

  LV: The Smallest and Widest types: 32 / 32 bits.
  LV: The Widest register is: 256 bits.
  LV: Found an estimated cost of 0 for VF 1 For instruction:   %indvars.iv25 = phi i64 [ 0, %entry ], [ %indvars.iv.next26, %for.body ]
  LV: Found an estimated cost of 0 for VF 1 For instruction:   %arrayidx = getelementptr inbounds [1600 x i32], [1600 x i32]* %a, i64 0, i64 %indvars.iv25
  LV: Found an estimated cost of 0 for VF 1 For instruction:   %2 = trunc i64 %indvars.iv25 to i32
  LV: Found an estimated cost of 1 for VF 1 For instruction:   store i32 %2, i32* %arrayidx, align 4
  LV: Found an estimated cost of 1 for VF 1 For instruction:   %indvars.iv.next26 = add nuw nsw i64 %indvars.iv25, 1
  LV: Found an estimated cost of 1 for VF 1 For instruction:   %exitcond27 = icmp eq i64 %indvars.iv.next26, 1600
  LV: Found an estimated cost of 0 for VF 1 For instruction:   br i1 %exitcond27, label %for.cond.cleanup, label %for.body
  LV: Scalar loop costs: 3.
  LV: Found an estimated cost of 0 for VF 2 For instruction:   %indvars.iv25 = phi i64 [ 0, %entry ], [ %indvars.iv.next26, %for.body ]
  LV: Found an estimated cost of 0 for VF 2 For instruction:   %arrayidx = getelementptr inbounds [1600 x i32], [1600 x i32]* %a, i64 0, i64 %indvars.iv25
  LV: Found an estimated cost of 0 for VF 2 For instruction:   %2 = trunc i64 %indvars.iv25 to i32
  LV: Found an estimated cost of 2 for VF 2 For instruction:   store i32 %2, i32* %arrayidx, align 4
  LV: Found an estimated cost of 1 for VF 2 For instruction:   %indvars.iv.next26 = add nuw nsw i64 %indvars.iv25, 1
  LV: Found an estimated cost of 1 for VF 2 For instruction:   %exitcond27 = icmp eq i64 %indvars.iv.next26, 1600
  LV: Found an estimated cost of 0 for VF 2 For instruction:   br i1 %exitcond27, label %for.cond.cleanup, label %for.body
  LV: Vector loop of width 2 costs: 2.
  LV: Found an estimated cost of 0 for VF 4 For instruction:   %indvars.iv25 = phi i64 [ 0, %entry ], [ %indvars.iv.next26, %for.body ]
  LV: Found an estimated cost of 0 for VF 4 For instruction:   %arrayidx = getelementptr inbounds [1600 x i32], [1600 x i32]* %a, i64 0, i64 %indvars.iv25
  LV: Found an estimated cost of 0 for VF 4 For instruction:   %2 = trunc i64 %indvars.iv25 to i32
  LV: Found an estimated cost of 4 for VF 4 For instruction:   store i32 %2, i32* %arrayidx, align 4
  LV: Found an estimated cost of 1 for VF 4 For instruction:   %indvars.iv.next26 = add nuw nsw i64 %indvars.iv25, 1
  LV: Found an estimated cost of 1 for VF 4 For instruction:   %exitcond27 = icmp eq i64 %indvars.iv.next26, 1600
  LV: Found an estimated cost of 0 for VF 4 For instruction:   br i1 %exitcond27, label %for.cond.cleanup, label %for.body
  LV: Vector loop of width 4 costs: 1.
  ...
  LV: Selecting VF: 8.
  LV: The target has 32 registers
  LV(REG): Calculating max register usage:
  LV(REG): At #0 Interval # 0
  LV(REG): At #1 Interval # 1
  LV(REG): At #2 Interval # 2
  LV(REG): At #4 Interval # 1
  LV(REG): At #5 Interval # 1
  LV(REG): VF = 8

The problem is that the cost model here is not wrong, exactly. Since all of
these operations are scalarized, their cost (aside from the uniform ones) are
indeed VF*(scalar cost), just as the model suggests. In fact, the larger the VF
picked, the lower the relative overhead from the loop itself (and the
induction-variable update and check), and so in a sense, picking the largest VF
here is the right thing to do.

The problem is that vectorizing like this, where all of the vectors will be
scalarized in the backend, isn't really vectorizing, but rather interleaving.
By itself, this would be okay, but then the vectorizer itself also interleaves,
and that's where the problem manifests itself. There's aren't actually enough
scalar registers to support the normal interleave factor multiplied by a factor
of VF (8 in this example). In other words, the problem with this is that our
register-pressure heuristic does not account for scalarization.

While we might want to improve our register-pressure heuristic, I don't think
this is the right motivating case for that work. Here we have a more-basic
problem: The job of the vectorizer is to vectorize things (interleaving aside),
and if the IR it generates won't generate any actual vector code, then
something is wrong. Thus, if every type looks like it will be scalarized (i.e.
will be split into VF or more parts), then don't consider that VF.

This is not a problem specific to PPC/QPX, however. The problem comes up under
SSE on x86 too, and as such, this change fixes PR26837 too. I've added Sanjay's
reduced test case from PR26837 to this commit.

Differential Revision: http://reviews.llvm.org/D18537

llvm-svn: 264904