[ScalarizeMaskedMemIntrin] Add support for scalarizing expandload and compressstore...

[ScalarizeMaskedMemIntrin] Add support for scalarizing expandload and compressstore intrinsics.

This adds support for scalarizing these intrinsics as well the X86TargetTransformInfo support to avoid scalarizing them in the cases X86 can handle.

I've omitted handling special cases for constant masks for this first pass. Though CodeGenPrepare can constant fold the branch conditions and remove some of the control flow anyway.

Fixes PR40994 and is covers most of PR3666. Might want to implement constant masks to close that.

Differential Revision: https://reviews.llvm.org/D59180

llvm-svn: 356687