[llvm] [AMDGPU] Assert that we can find subregs in copyPhysReg. NFC. (PR #70332)

[Jay Foad via llvm-commits]

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@@ -742,23 +742,27 @@ static void expandSGPRCopy(const SIInstrInfo &TII, MachineBasicBlock &MBB,
 
   for (unsigned Idx = 0; Idx < BaseIndices.size(); ++Idx) {
     int16_t SubIdx = BaseIndices[Idx];
-    Register Reg = RI.getSubReg(DestReg, SubIdx);
+    Register DestSubReg = RI.getSubReg(DestReg, SubIdx);
+    Register SrcSubReg = RI.getSubReg(SrcReg, SubIdx);
+    assert(DestSubReg && SrcSubReg && "Failed to find subregs!");
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jayfoad wrote:

I did wonder about adding a new variant of `getSubReg(MCRegister, unsigned)` that does the assert for you, but I couldn't think of a good name for it.

https://github.com/llvm/llvm-project/pull/70332