[mips] Implement sgt/sgtu pseudo instructions with immediate operand

The `sgt/sgtu Dst, Src1, Src2/Imm` pseudo instructions set register
`Dst` to 1 if register `Src1` is greater than `Src2/Imm` and to 0 otherwise.

Differential Revision: https://reviews.llvm.org/D64313

llvm-svn: 365475