Move mnemonicIsValid to Mips target.

Summary:
The Mips target is the only user of mnemonicIsValid. This patch
moves this method from AsmMatcherEmitter.cpp to MipsAsmParser.cpp,
getting rid of the method in all other targets where it generated
warnings about an unused function.

Patch by Gonsolo.

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: sdardis

Differential Revision: https://reviews.llvm.org/D28748

llvm-svn: 294400