[X86][MC] Fix bracket expression parsing in intel-style assembly.

Only perform struct field check on Identifier tokens.

Fixes PR28547.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22361

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275445 91177308-0d34-0410-b5e6-96231b3b80d8