Revert "[RISCV] Remove dead early exit in performCombineVMergeAndVOps. NFC"

This reverts commit cee60a8.

This change is not NFC because there is also an additional check for
isImplicitDef in the branch above.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -3545,6 +3545,9 @@ 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