[X86] Teach X86MCodeEmitter to properly encode zmm16-zmm31 as index register...

[X86] Teach X86MCodeEmitter to properly encode zmm16-zmm31 as index register to vgatherpf/vscatterpf.

We need to encode bit 4 into the EVEX.V' bit. We do this right
for regular gather/scatter which use either MRMSrcMem or MRMDestMem
formats.  The prefetches use MRM*m formats.

Fixes an issue recently added to PR36202.

llvm-svn: 374849