[mips] Place certain 64 bit FPU instructions in their own decoder namespace

[mips] Place certain 64 bit FPU instructions in their own decoder namespace

Previously, instructions that were defined to use the FGR64 register class
were associated with the Mips64 table which was incorrect.

Reviewers: nitesh.jain, atanasyan

Differential Revision: https://reviews.llvm.org/D38454

llvm-svn: 314976