[RISCV] Don't set AVL if only zeroness is demanded (#74049)

This refactors the logic in transferBefore so that we're moving in the
direction of "keep the existing Info, only change what is needed".

For the sake of review there are two commits in this PR: The former is
needed to make the latter an NFC commit. Neither introduce any test
diffs but the former is not technically NFC, hence why I did not
precommit it.

- [RISCV] Preserve AVL when previous info is ratio only in
transferBefore
- [RISCV] Don't change AVL if only zeroness is demanded. NFC

Author: lukel97

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -1059,14 +1059,23 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
     return;
 
   const VSETVLIInfo PrevInfo = Info;
-  if (Info.hasSEWLMULRatioOnly() || !Info.isValid() || Info.isUnknown())
+  if (!Info.isValid() || Info.isUnknown())
     Info = NewInfo;
 
   DemandedFields Demanded = getDemanded(MI, MRI, ST);
   const VSETVLIInfo IncomingInfo =
       adjustIncoming(PrevInfo, NewInfo, Demanded, MRI);
 
-  if (Demanded.usedVL())
+  // If MI only demands that VL has the same zeroness, we only need to set the
+  // AVL if the zeroness differs.  This removes a vsetvli entirely if the types
+  // match or allows use of cheaper avl preserving variant if VLMAX doesn't
+  // change. If VLMAX might change, we couldn't use the 'vsetvli x0, x0, vtype"
+  // variant, so we avoid the transform to prevent extending live range of an
+  // avl register operand.
+  // TODO: We can probably relax this for immediates.
+  bool EquallyZero = IncomingInfo.hasEquallyZeroAVL(PrevInfo, *MRI) &&
+                     IncomingInfo.hasSameVLMAX(PrevInfo);
+  if (Demanded.VLAny || (Demanded.VLZeroness && !EquallyZero))
     Info.setAVL(IncomingInfo);
 
   Info.setVTYPE(
@@ -1079,6 +1088,14 @@ void RISCVInsertVSETVLI::transferBefore(VSETVLIInfo &Info,
           IncomingInfo.getTailAgnostic(),
       (Demanded.MaskPolicy ? IncomingInfo : Info).getMaskAgnostic() ||
           IncomingInfo.getMaskAgnostic());
+
+  // If we only knew the sew/lmul ratio previously, replace the VTYPE but keep
+  // the AVL.
+  if (Info.hasSEWLMULRatioOnly()) {
+    VSETVLIInfo RatiolessInfo = IncomingInfo;
+    RatiolessInfo.setAVL(Info);
+    Info = RatiolessInfo;
+  }
 }
 
 static VSETVLIInfo adjustIncoming(VSETVLIInfo PrevInfo, VSETVLIInfo NewInfo,
@@ -1097,20 +1114,6 @@ static VSETVLIInfo adjustIncoming(VSETVLIInfo PrevInfo, VSETVLIInfo NewInfo,
     Demanded.LMUL = true;
   }
 
-  // If we only demand VL zeroness (i.e. vmv.s.x and vmv.x.s), then there are
-  // only two behaviors, VL = 0 and VL > 0. We can discard the user requested
-  // AVL and just use the last one if we can prove it equally zero. This
-  // removes a vsetvli entirely if the types match or allows use of cheaper avl
-  // preserving variant if VLMAX doesn't change. If VLMAX might change, we
-  // couldn't use the 'vsetvli x0, x0, vtype" variant, so we avoid the transform
-  // to prevent extending live range of an avl register operand.
-  // TODO: We can probably relax this for immediates.
-  if (Demanded.VLZeroness && !Demanded.VLAny && PrevInfo.isValid() &&
-      PrevInfo.hasEquallyZeroAVL(Info, *MRI) && Info.hasSameVLMAX(PrevInfo)) {
-    Info.setAVL(PrevInfo);
-    Demanded.demandVL();
-  }
-
   return Info;
 }