[X86] fold select to mask instructions.

[LuoYuanke]

Summary

When avx512 is available the lhs operand of select instruction can be
folded with mask instruction, while the rhs operand can't. This patch is
to commute the lhs and rhs of the select instruction to create the
opportunity of folding.

[RKSimon]

llvm/lib/Target/X86/X86ISelLowering.cpp

20028

Do we actually need canCombineAsMaskOperation to take 2 input operands? Its other use in lowerVECTOR_SHUFFLE looks like it could be split to call it twice.

[LuoYuanke]

llvm/lib/Target/X86/X86ISelLowering.cpp

20028

Taking 1 input operand looks better.

[LuoYuanke]

NFC. Address Simon's comments.

[RKSimon]

LGTM - please commit the canCombineAsMaskOperation/lowerVECTOR_SHUFFLE refactor first as a pre-commit and then the commuteSelect patch afterward.

[LuoYuanke]

Rebase and add code comments.