[mips] Rename MipsAsmParser functions to conform to the LLVM Coding Standards. No functional changes.

Summary: There are still some functions which should be renamed, but they are inherited from the generic MC classes.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D5068

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217145 91177308-0d34-0410-b5e6-96231b3b80d8