[llvm] [DebugInfo][ConstraintElimination] Fix debug value loss in replacing comparisons with the speculated constants (PR #136839)

[Shan Huang via llvm-commits]

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@@ -1454,8 +1455,20 @@ static bool checkAndReplaceCondition(
       return ShouldReplace;
     });
     NumCondsRemoved++;
-    if (Cmp->use_empty())
+    if (Cmp->use_empty()) {
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Apochens wrote:

🤔 It seems that this is the right way. The condition in `replaceUesWithIf` specifies where we can use the speculated constant, so there could be cases that non-dbg uses still exist after the optimization. I have modified the return value to `%t.1` in test `salvage-dbg-values-replaced-by-constant.ll` to demostrate such a case.

https://github.com/llvm/llvm-project/pull/136839