[llvm] [RISCV] Unify vsetvli compatibility logic in forward and backwards passes (PR #71657)

[Luke Lau via llvm-commits]

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@@ -1495,14 +1472,31 @@ void RISCVInsertVSETVLI::doLocalPostpass(MachineBasicBlock &MBB) {
         ToDelete.push_back(&MI);
         // Leave NextMI unchanged
         continue;
-      } else if (canMutatePriorConfig(MI, *NextMI, Used)) {
+      } else if (NextInfo.isCompatible(Used, getInfoForVSETVLI(MI), *MRI,
+                                       true)) {
         if (!isVLPreservingConfig(*NextMI)) {
           MI.getOperand(0).setReg(NextMI->getOperand(0).getReg());
           MI.getOperand(0).setIsDead(false);
+
+          MachineOperand &AVL = MI.getOperand(1);
+          // If the old AVL was only used by MI, it's dead.
+          if (AVL.isReg() && AVL.getReg().isVirtual() &&
+              MRI->hasOneNonDBGUse(AVL.getReg()))
+            MRI->getVRegDef(AVL.getReg())->eraseFromParent();
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lukel97 wrote:

This looks like it could have been pulled out into a separate patch but it has no effect on main, because we currently don't consider these cases as compatible

https://github.com/llvm/llvm-project/pull/71657