[LoopVectorize] When tail-folding, don't always predicate uniform loads

[LoopVectorize] When tail-folding, don't always predicate uniform loads

In VPRecipeBuilder::handleReplication if we believe the instruction
is predicated we then proceed to create new VP region blocks even
when the load is uniform and only predicated due to tail-folding.

I have updated isPredicatedInst to avoid treating a uniform load as
predicated when tail-folding, which means we can do a single scalar
load and a vector splat of the value.

Tests added here:

  Transforms/LoopVectorize/AArch64/tail-fold-uniform-memops.ll

Differential Revision: https://reviews.llvm.org/D112552