[mips] Use early exit in MipsAsmParser::matchCPURegisterName(). NFC.

Patch by Vasileios Kalintiris.

Differential Revision: http://reviews.llvm.org/D5270



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217774 91177308-0d34-0410-b5e6-96231b3b80d8