[llvm] [SCEV] Collect and merge loop guards through PHI nodes with multiple incoming values (PR #113915)

[Florian Hahn via llvm-commits]

================
@@ -15596,6 +15603,66 @@ ScalarEvolution::LoopGuards::collect(const Loop *L, ScalarEvolution &SE) {
     Terms.emplace_back(LoopEntryPredicate->getCondition(),
                        LoopEntryPredicate->getSuccessor(0) == Pair.second);
   }
+  // Finally, if we stopped climbing the predecessor chain because
+  // there wasn't a unique one to continue, try to collect conditions
+  // for PHINodes by recursively following all of their incoming
+  // blocks and try to merge the found conditions to build a new one
+  // for the Phi.
+  if (Pair.second->hasNPredecessorsOrMore(2)) {
+    for (auto &Phi : Pair.second->phis()) {
+      if (!SE.isSCEVable(Phi.getType()))
+        continue;
+
+      using MinMaxPattern = std::pair<const SCEVConstant *, SCEVTypes>;
+      auto GetMinMaxConst = [&SE, &VisitedBlocks, &Pair,
+                             &Phi](unsigned int In) -> MinMaxPattern {
----------------
fhahn wrote:

```suggestion
                             &Phi](unsigned In) -> MinMaxPattern {
```

https://github.com/llvm/llvm-project/pull/113915