[RISCV] Fold vmerge into its ops with smaller VL if known

Currently when folding vmerge into its operands, we stop if the VLs aren't
identical.  However since the body of (vmerge (vop)) is the intersection of
vmerge and vop's bodies, we can use the smaller of the two VLs if we know it
ahead of time.  This patch relaxes the constraint on VL if they are both
constants, or if either of them are VLMAX.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D155071