[llvm] [Utils] Fix incorrect LCSSA PHI nodes when splitting critical edges with MergeIdenticalEdges (PR #131744)

[Nikita Popov via llvm-commits]

================
@@ -438,6 +438,63 @@ define void @crit_edge(i1 %cond0, i1 %cond1) {
   EXPECT_TRUE(PDT.verify());
 }
 
+TEST(BasicBlockUtils, SplitLoopCriticalEdge) {
+  LLVMContext C;
+  std::unique_ptr<Module> M = parseIR(C, R"IR(
+declare dso_local i1 @predicate(ptr noundef %p)
+
+define dso_local ptr @Parse(ptr noundef %gp) {
+entry:
+  br label %for.inc
+
+for.inc:
+  %phi = phi ptr [ %gp, %entry ], [ %cp, %while.cond ], [ %cp, %while.cond ]
+  %cond = call i1 @predicate(ptr noundef %phi)
+  %inc= getelementptr inbounds i8, ptr %phi, i64 1
----------------
nikic wrote:

```suggestion
  %inc = getelementptr inbounds i8, ptr %phi, i64 1
```

https://github.com/llvm/llvm-project/pull/131744