[mips] Make FileCheck prefixes match the CPU they check

Summary:
For example: s/MIPS32-EB-AS/MIPS32R2-EB-AS/ since it is for MIPS32r2 not MIPS32.

This reduces the noise in my next Clang patch.

Differential Revision: http://reviews.llvm.org/D4271

llvm-svn: 212177