[X86] Add encoding information for the AVX5124FMAPS and AVX5124VNNIW...

[X86] Add encoding information for the AVX5124FMAPS and AVX5124VNNIW instructions so they can be assembled and disassembled.

These instructions are unusual in that they operate on 4 consecutive registers so supporting them in codegen will be more difficult than normal.

Includes an assembler check to warn if the source register is not the first register of a 4 register group.

llvm-svn: 333812