[RISCV] Merge vmerge.vvm and unmasked intrinsic with VLMAX vector length.

The motivation of this patch is to lower the IR pattern
(vp.merge mask, (add x, y), false, vl) to
(PseudoVADD_VV_<LMUL>_MASK false, x, y, mask, vl).

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D131841