[llvm] [SROA] Unfold gep of index select (PR #80983)

[Arthur Eubanks via llvm-commits]

================
@@ -3937,30 +3937,63 @@ class AggLoadStoreRewriter : public InstVisitor<AggLoadStoreRewriter, bool> {
     return false;
   }
 
-  // Fold gep (select cond, ptr1, ptr2) => select cond, gep(ptr1), gep(ptr2)
+  // Fold gep (select cond, ptr1, ptr2), idx
+  //   => select cond, gep(ptr1, idx), gep(ptr2, idx)
+  // and  gep ptr, (select cond, idx1, idx2)
+  //   => select cond, gep(ptr, idx1), gep(ptr, idx2)
   bool foldGEPSelect(GetElementPtrInst &GEPI) {
-    if (!GEPI.hasAllConstantIndices())
-      return false;
+    // Check whether the GEP has exactly one select operand and all indices
+    // will become constant after the transform.
+    auto IsValidOp = [](Value *Op) {
+      return Op->getType()->isPointerTy() || isa<ConstantInt>(Op);
+    };
+
+    SelectInst *Sel = nullptr;
+    for (Value *Op : GEPI.operands()) {
+      if (auto *SI = dyn_cast<SelectInst>(Op)) {
----------------
aeubanks wrote:

```
    if (!Sel || !IsValidOp(Sel->getTrueValue()) ||
        !IsValidOp(Sel->getFalseValue()))
      return false;
```
below bails if either op is not a constant int

https://github.com/llvm/llvm-project/pull/80983