[X86] Make AVX512_512_SET0 XMM16-31 lower to 128-bit XOR when AVX512VL is enabled...

[X86] Make AVX512_512_SET0 XMM16-31 lower to 128-bit XOR when AVX512VL is enabled. Use 128-bit VLX instruction when VLX is enabled.

Unfortunately, this weakens our ability to do domain fixing when AVX512DQ is not enabled, but it is consistent with our 256-bit behavior.

Maybe we should add custom handling to domain fixing to allow EVEX integer XOR/AND/OR/ANDN to switch to VEX encoded fp instructions if the high registers aren't being used?

llvm-svn: 316978