[llvm] [NFC][RISCV] Keep AVLReg define instr inside VSETVLInfo (PR #89180)

[Luke Lau via llvm-commits]

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@@ -478,6 +480,8 @@ class VSETVLIInfo {
     State = AVLIsImm;
   }
 
+  void setAVLDefMI(const MachineInstr *DefMI) { AVLDefMI = DefMI; }
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lukel97 wrote:

I've done this part in #89964, but with the existing virtual register form

https://github.com/llvm/llvm-project/pull/89180