[llvm] [RISCV] Move RISCVInsertVSETVLI to after phi elimination (PR #91440)

[Luke Lau via llvm-commits]

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@@ -493,17 +501,17 @@ class VSETVLIInfo {
     assert(hasAVLImm());
     return AVLImm;
   }
-  const MachineInstr &getAVLDefMI() const {
-    assert(hasAVLReg() && AVLRegDef.DefMI);
-    return *AVLRegDef.DefMI;
+  const VNInfo *getAVLVNInfo() const {
+    assert(hasAVLReg());
+    return AVLRegDef.AVLVNInfo;
   }
 
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lukel97 wrote:

What were previously PHI node MachineInstrs will now be VNInfos with a def that points to the block boundary slot, so we can't get a defining MI. But the helper still makes sense I think, we can just have it return null.

And upon closer inspection actually, the only places where we actually get the defining MI, is to check if it's a vsetvli or an addi. 

https://github.com/llvm/llvm-project/pull/91440