[llvm] Add non-null check before accessing pointer (PR #83459)

[Matt Arsenault via llvm-commits]

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@@ -1054,7 +1054,7 @@ void SIFoldOperands::foldOperand(
       // Don't fold if OpToFold doesn't hold an aligned register.
       const TargetRegisterClass *RC =
           TRI->getRegClassForReg(*MRI, OpToFold.getReg());
-      if (TRI->hasVectorRegisters(RC) && OpToFold.getSubReg()) {
+      if (RC && TRI->hasVectorRegisters(RC) && OpToFold.getSubReg()) {
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arsenm wrote:

There's no context that can reach here where this can fail 

https://github.com/llvm/llvm-project/pull/83459