[llvm] [HashRecognize] Fix compiler hang on dense use-def chain (PR #211711)

[Ramkumar Ramachandra via llvm-commits]

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@@ -0,0 +1,171 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes='print<hash-recognize>' -disable-output %s 2>&1 | FileCheck %s
+
+; This loop hangs if digRecurrence doesn't keep track of previously visited
+; instructions since most instructions have multiple uses.
+define void @dense_use_def_chain() {
+; CHECK-LABEL: 'dense_use_def_chain'
+; CHECK-NEXT:  Did not find a hash algorithm
+; CHECK-NEXT:  Reason: Unable to find conditional recurrence
+;
+bbl:
+  %load = load i32, ptr null, align 4
+  %sitofp = sitofp i32 %load to float
+  br label %bbl1
+
+bbl1:                                             ; preds = %bbl1, %bbl
+  %phi = phi i32 [ 0, %bbl ], [ %add, %bbl1 ]
+  %phi2 = phi float [ poison, %bbl ], [ %select144, %bbl1 ]
----------------
artagnon wrote:

```suggestion
  %ind = phi float [ 0.0, %bbl ], [ %ind.next, %bbl1 ]
```

https://github.com/llvm/llvm-project/pull/211711