[RISCV] Commute True in foldVMergeToMask (#156499)
In order to fold a vmerge into a pseudo, the pseudo's passthru needs to
be the same as vmerge's false operand.
If they don't match we can try and commute the instruction if possible,
e.g. here we can commute v9 and v8 to fold the vmerge:
vsetvli zero, a0, e32, m1, ta, ma
vfmadd.vv v9, v10, v8
vsetvli zero, zero, e32, m1, tu, ma
vmerge.vvm v8, v8, v9, v0
vsetvli zero, a0, e32, m1, tu, mu
vfmacc.vv v8, v9, v10, v0.t
Previously this wasn't possible because we did the peephole in
SelectionDAG, but now that it's been migrated to MachineInstr in #144076
we can reuse the commuting infrastructure in TargetInstrInfo.
This fixes the extra vmv.v.v in the "mul" example here:
https://github.com/llvm/llvm-project/issues/123069#issuecomment-3137997141
It should also allow us to remove the isel patterns described in #141885
later.
