[InstCombine] try harder to form rotate (funnel shift) (PR20750)

[InstCombine] try harder to form rotate (funnel shift) (PR20750)

We have a similar match for patterns ending in a truncate. This
should be ok for all targets because the default expansion would
still likely be better from replacing 2 'and' ops with 1.

Attempt to show the logic equivalence in Alive (which doesn't
currently have funnel-shift in its vocabulary AFAICT):

  %shamt = zext i8 %i to i32
  %m = and i32 %shamt, 31
  %neg = sub i32 0, %shamt
  %and4 = and i32 %neg, 31
  %shl = shl i32 %v, %m
  %shr = lshr i32 %v, %and4
  %or = or i32 %shr, %shl
  =>
  %a = and i8 %i, 31
  %shamt2 = zext i8 %a to i32
  %neg2 = sub i32 0, %shamt2
  %and4 = and i32 %neg2, 31
  %shl = shl i32 %v, %shamt2
  %shr = lshr i32 %v, %and4
  %or = or i32 %shr, %shl

https://rise4fun.com/Alive/V9r

llvm-svn: 360605