[mips] Fix test mips64fpldst.ll with machine verifier enabled

[mips] Fix test mips64fpldst.ll with machine verifier enabled

Removed micro mips register classes for gp initialization because gp initialization uses pure mips64 instruction. Even when compiling for micro mips, gp initialization can be done with pure mips64 instructions.

Reviewed by Simon Dardis
Differential: D32286

llvm-svn: 301394