[mips] Hoist Parser.Lex() calls out of MatchAnyRegisterNameWithoutDollar()

Summary:
No functional change

Depends on D3222

Reviewers: matheusalmeida, vmedic

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3232

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205295 91177308-0d34-0410-b5e6-96231b3b80d8