[CSInfo][MIPS] Don't describe parameters loaded by sub/super reg copy

When describing parameter value loaded by a COPY instruction, consider
case where needed Reg value is a sub- or super- register of the COPY
instruction's destination register. Without this patch, compile process
will crash with the assertion "TargetInstrInfo::describeLoadedValue
can't describe super- or sub-regs for copy instructions".

Patch by Nikola Tesic

Differential revision: https://reviews.llvm.org/D82000