[mips] Use MFHC1 when it is available (MIPS32r2 and later) for both FP32 and FP64 moves

Summary:
This is similar to r210771 which did the same thing for MTHC1.

Also corrected MTHC1_D32 and MTHC1_D64 which used AFGR64 and FGR64 on the
wrong definitions.

Differential Revision: http://reviews.llvm.org/D4483