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

[Martin Wehking 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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MartinWehking wrote:

Is `getRegClassForReg` always returning a non-null pointer for every possible context?
When looking at the body of the function:
`return Reg.isVirtual() ? MRI.getRegClass(Reg) : getPhysRegBaseClass(Reg); `
I spotted that getPhysRegBaseClass(Reg) might return a nullptr.

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