[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


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212177 91177308-0d34-0410-b5e6-96231b3b80d8