[llvm] [InstCombine] Optimistically allow multiple shufflevector uses in foldOpPhi (PR #114278)

[Yingwei Zheng via llvm-commits]

================
@@ -1773,17 +1773,33 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN) {
   if (NumPHIValues == 0)
     return nullptr;
 
-  // We normally only transform phis with a single use.  However, if a PHI has
-  // multiple uses and they are all the same operation, we can fold *all* of the
-  // uses into the PHI.
+  // We normally only transform phis with a single use.
+  bool AllUsesIdentical = false;
+  bool MultipleUses = false;
   if (!PN->hasOneUse()) {
-    // Walk the use list for the instruction, comparing them to I.
+    // Exceptions:
+    //   - All uses are identical.
+    //   - All uses are shufflevector instructions that fully simplify; this
+    //     helps interleave -> phi -> 2x de-interleave+de patterns.
+    if (isa<ShuffleVectorInst>(I)) {
+      MultipleUses = true;
+    }
----------------
dtcxzyw wrote:

```suggestion
    MultipleUses = isa<ShuffleVectorInst>(I);
```

https://github.com/llvm/llvm-project/pull/114278