[llvm] [PHIElimination] Handle subranges in LiveInterval updates (PR #69429)

[Jay Foad via llvm-commits]

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@@ -139,6 +139,10 @@ class VirtRegMap;
       return LI;
     }
 
+    LiveInterval &getOrCreateEmptyInterval(Register Reg) {
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jayfoad wrote:

Should really have a doxygen comment, even if it's obvious.

https://github.com/llvm/llvm-project/pull/69429