[AVX512] Pull pattern for subvector extract into the instruction definition

No functional change.

I initially thought that pulling the Pat<> into the instruction pattern was
not possible because it was doing a transform on the index in order to convert
it from a per-element (extract_subvector) index into a per-chunk (vextract*x4)
index.

Turns out this also works inside the pattern because the vextract_extract
PatFrag has an OperandTransform EXTRACT_get_vextract{128,256}_imm, so the
index in $idx goes through the same conversion.

The existing test CodeGen/X86/avx512-insert-extract.ll extended in the
previous commit provides coverage for this change.

llvm-svn: 218480