[RISCV] Teach performCombineVMergeAndVOps to combine unmasked TU vpmerge with a masked MU TA op.

We can form a MU TU operation and remove the merge if they use the
same merge value.

My primary interest was a case involving VP intrinsics from our downstream,
but it requires another optimization that isn't in upstream yet. So I've used
RVV intrinsics to get the desired instructions.

Co-authored-by: Nitin John Raj <nitin.raj@sifive.com>

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D145272