[RISCV] Update performCombineVMergeAndVOps comments. NFC (#78472)

The current comment was written whenever we had separate TU/TA variants
for
each pseudo, and hasn't been accurate for a while.
This method has grown rather complicated over time so rather than
enumerate all
the different possible cases now (which must be a lot), this updates the
comment to list the different rules that are required for us to be able
to fold
a vmerge.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -3458,19 +3458,27 @@ static unsigned GetVMSetForLMul(RISCVII::VLMUL LMUL) {
   llvm_unreachable("Unknown VLMUL enum");
 }
 
-// Try to fold away VMERGE_VVM instructions. We handle these cases:
-// -Masked TU VMERGE_VVM combined with an unmasked TA instruction instruction
-//  folds to a masked TU instruction. VMERGE_VVM must have have merge operand
-//  same as false operand.
-// -Masked TA VMERGE_VVM combined with an unmasked TA instruction fold to a
-//  masked TA instruction.
-// -Unmasked TU VMERGE_VVM combined with a masked MU TA instruction folds to
-//  masked TU instruction. Both instructions must have the same merge operand.
-//  VMERGE_VVM must have have merge operand same as false operand.
-// Note: The VMERGE_VVM forms above (TA, and TU) refer to the policy implied,
-// not the pseudo name.  That is, a TA VMERGE_VVM can be either the _TU pseudo
-// form with an IMPLICIT_DEF passthrough operand or the unsuffixed (TA) pseudo
-// form.
+// Try to fold away VMERGE_VVM instructions into their true operands:
+//
+// %true = PseudoVADD_VV ...
+// %x = PseudoVMERGE_VVM %false, %false, %true, %mask
+// ->
+// %x = PseudoVADD_VV_MASK %false, ..., %mask
+//
+// We can only fold if vmerge's merge operand, vmerge's false operand and
+// %true's merge operand (if it has one) are the same. This is because we have
+// to consolidate them into one merge operand in the result.
+//
+// If %true is masked, then we can use its mask instead of vmerge's if vmerge's
+// mask is all ones.
+//
+// We can also fold a VMV_V_V into its true operand, since it is equivalent to a
+// VMERGE_VVM with an all ones mask.
+//
+// The resulting VL is the minimum of the two VLs.
+//
+// The resulting policy is the effective policy the vmerge would have had,
+// i.e. whether or not it's merge operand was implicit-def.
 bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
   SDValue Merge, False, True, VL, Mask, Glue;
   // A vmv.v.v is equivalent to a vmerge with an all-ones mask.
@@ -3530,26 +3538,26 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
   if (Info->MaskAffectsResult && Mask && !usesAllOnesMask(Mask, Glue))
     return false;
 
+  // If True has a merge operand then it needs to be the same as vmerge's False,
+  // since False will be used for the result's merge operand.
   if (HasTiedDest && !isImplicitDef(True->getOperand(0))) {
     // The vmerge instruction must be TU.
     // FIXME: This could be relaxed, but we need to handle the policy for the
     // resulting op correctly.
     if (isImplicitDef(Merge))
       return false;
     SDValue MergeOpTrue = True->getOperand(0);
-    // Both the vmerge instruction and the True instruction must have the same
-    // merge operand.
     if (False != MergeOpTrue)
       return false;
   }
 
+  // If True is masked then the vmerge must have an all 1s mask, since we're
+  // going to keep the mask from True.
   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
     // undef merge operand.
     if (Mask && !usesAllOnesMask(Mask, Glue))