[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

llvm-svn: 205295