[X86] Merge the last of the useBWIRegs() section into the useAVX512Regs()...

[X86] Merge the last of the useBWIRegs() section into the useAVX512Regs() section of the X86TargetLowering constructor. NFC

This section is the remnant of how this code was structured before
we made v32i16/v64i8 legal types with avx512f when not restricting
to 256 bit vectors. Now that there are just a few items left,
merge them near similar things in the other section.