TableGen: Work around assert on Mips register definitions

This would hit the "Biggest class wasn't first" assert in
getMatchingSubClassWithSubRegs in a future patch for EXTRACT_SUBREG
handling.

Mips defines 4 identical register classes (MSA128B, MSA128H, MSA128BW,
MSA128D). These have the same set of registers, and only differ by the
isel type. I believe this is an ill formed way of defining registers,
that probably is just to work around the inconvenience of mixing
different types in a single register class in DAG patterns.

Since these all have the same size, they would all sort to the
beginning, but you would not necessarily get the same super register
at the front as the assert enforces. Breaking the ambiguity by also
sorting by name doesn't work, since each of these register classes all
want to be first. Force sorting of the original register class if the
size is the same.

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