[X86] Teach the assembly parser to warn on duplicate registers in gather instructions.

Fixes PR32238.

Differential Revision: https://reviews.llvm.org/D39077

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316700 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed