[X86] Add explicit mc-encoding checks to X86/viabs.ll. NFC.

Add explicit mc-encoding checks showing that the AVX512VL ABS intrinsics are actually mapped to EVEX encoding.
This is a pre-commit for a soon to come patch which will lower x86 target specific ABS intrinsics to IR.

Differential Revision: https://reviews.llvm.org/D37688

llvm-svn: 313131