[SLP] Use SCEV to sort memory accesses.

[SLP] Use SCEV to sort memory accesses.

This generalizes memory access sorting to use differences between SCEVs,
instead of relying on constant offsets. That allows us to properly do
SLP vectorization of non-sequentially ordered loads within loops bodies.

Differential Revision: https://reviews.llvm.org/D29425

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294027 91177308-0d34-0410-b5e6-96231b3b80d8