[RISCV] Remove dead early exit in performCombineVMergeAndVOps. NFC

Because a masked instruction always has a tied dest (because it has a
passthru), the early exit if Merge is an implicit def will have always already
been taken in the branch above that handles HasTiedDest.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -3545,9 +3545,6 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
 
   if (IsMasked) {
     assert(HasTiedDest && "Expected tied dest");
-    // The vmerge instruction must be TU.
-    if (isImplicitDef(Merge))
-      return false;
     // The vmerge instruction must have an all 1s mask since we're going to keep
     // the mask from the True instruction.
     // FIXME: Support mask agnostic True instruction which would have an