[CodeGenPrepare][x86] shift both sides of a vector select when profitable

This is based on the example/discussion in PR37428:
https://bugs.llvm.org/show_bug.cgi?id=37428

Proper vector shift instructions don't appear until AVX2, so we may generate several
extra instructions within a loop trying to compensate for that. It's difficult to
recover from that shift expansion later than this, so use the existing TLI hook and
splat analysis to enable better codegen.

This extends CGP functionality introduced with:
rL201655

Differential Revision: https://reviews.llvm.org/D63233

llvm-svn: 363511