[X86] Add -show-mc-encoding to the avx512-vec-cmp.ll test and add test case to...

[X86] Add -show-mc-encoding to the avx512-vec-cmp.ll test and add test case to show that we're failing to use the shorter pcmpeq encoding when the memory arguemnt is the first argument.

This can't be spotted without showing the encodings since they have the same mnemonic.

llvm-svn: 325455