New FR (FP reg at 25:21) encoding for the COP1X 4-register fused
multiply-adds MADD/MSUB/NMADD/NMSUB.S/.D. New GPR_AT_6 / GPR_AT_11
encodings (GPR in a vector-register slot, with correct GPR decode) for
MSA COPY_S/U (lane->GPR) and INSERT (GPR->lane). DSP two-register
PRECEQU/PRECEU (.PH.QBLA/QBRA) and REPLV (.PH/.QB). Control ops DI/EI and
RDHWR. 25 forms; spot-checked byte-exact vs llvm-mc and decode-clean; 281
tests green.