[mips][msa] Tidy up

lowerMSABinaryIntr, lowerMSABinaryImmIntr, lowerMSABranchIntr,
and lowerMSAUnaryIntr were trivially small functions. Inlined them into
their callers.

lowerMSASplat now takes its callers SDLoc instead of making a new one.

No functional change.