[X86] Print register names in .seh_* directives

[X86] Print register names in .seh_* directives

Also improve assembler parser register validation for .seh_ directives.
This requires moving X86-specific seh directive handling into the x86
backend, which addresses some assembler FIXMEs.

Differential Revision: https://reviews.llvm.org/D66625

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370533 91177308-0d34-0410-b5e6-96231b3b80d8