[mips] Use AsmLexer::peekTok() to resolve the conflict between $reg and $sym

Summary:
Parsing registers no longer consume the $ token before it's confirmed whether it really has a register or not, therefore it's no longer impossible to match symbols if registers were tried first.

Depends on D3232

Reviewers: matheusalmeida, vmedic

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3233