[llvm] [UniformityAnalysis] Skip CycleAnalysis on targets without branch divergence (PR #189948)

[Jay Foad via llvm-commits]

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@@ -1289,29 +1289,43 @@ GenericUniformityInfo<ContextT>::getFunction() const {
 }
 
 /// Whether \p V is divergent at its definition.
+/// A default-constructed instance (no analysis computed) reports everything
+/// as uniform, which is conservatively correct for non-divergent targets.
 template <typename ContextT>
 bool GenericUniformityInfo<ContextT>::isDivergent(ConstValueRefT V) const {
+  if (!DA)
+    return false;
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jayfoad wrote:

Suggestion:
```suggestion
  return DA && DA->isDivergent(V);
```
Similarly below.

https://github.com/llvm/llvm-project/pull/189948