Recommit r352255 "[SelectionDAG][X86] Don't use SEXTLOAD for promoting masked...

Recommit r352255 "[SelectionDAG][X86] Don't use SEXTLOAD for promoting masked loads in the type legalizer"

This did not cause the buildbot failure it was previously reverted for.

Original commit message:

I'm not sure why we were using SEXTLOAD. EXTLOAD seems more appropriate since we don't care about the upper bits.

This patch changes this and then modifies the X86 post legalization combine to emit a extending shuffle instead of a sign_extend_vector_inreg. Could maybe use an any_extend_vector_inre

On AVX512 targets I think we might be able to use a masked vpmovzx and not have to expand this at all.

llvm-svn: 352433