[mips] Extend range of register indexes accepted by cfcmsa/ctcmsa

The `cfcmsa` and `ctcmsa` instructions accept index of MSA control
register. The MIPS64 SIMD Architecture define eight MSA control
registers. But register index for `cfcmsa` and `ctcmsa` instructions
might be any number in 0..31 range. If the index is greater then 7,
`cfcmsa` writes zero to the destination registers and `ctcmsa` does
nothing [1].

[1] MIPS Architecture for Programmers Volume IV-j:
    The MIPS64 SIMD Architecture Module
https://www.mips.com/?do-download=the-mips64-simd-architecture-module

Differential Revision: https://reviews.llvm.org/D62597

llvm-svn: 362299