[llvm] [TableGen] Fix concatenation of subreg and artificial subregs (PR #114391)

[Jay Foad via llvm-commits]

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@@ -411,6 +410,9 @@ CodeGenRegister::computeSubRegs(CodeGenRegBank &RegBank) {
         Parts.push_back(getSubRegIndex(SR->ExplicitSubRegs[j]));
     }
 
+    if (Parts.size() < 2)
+      continue;
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jayfoad wrote:

Shouldn't we `continue` here if _any_ of the subreg indices of SR are artificial? (Either instead of or as well as the `Parts.size()` condition.) E.g. if SR had _two_ real subreg indices and one artificial one, we should give up?

https://github.com/llvm/llvm-project/pull/114391