[X86] Remove checks for FeatureAVX512 from the X86 assembly parser. Remove mcpu/mattr...

[X86] Remove checks for FeatureAVX512 from the X86 assembly parser. Remove mcpu/mattr from assembly test command lines.

Summary:
We should always be able to accept AVX512 registers and instructions in llvm-mc. The only subtarget mode that should be checked is 16-bit vs 32-bit vs 64-bit mode.

I've also removed all the mattr/mcpu lines from test RUN lines to be consistent with this. Most were due to AVX512, but a few were for other features.

Fixes PR36202

Reviewers: RKSimon, echristo, bkramer

Reviewed By: echristo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42824

llvm-svn: 324106