[llvm] [VPlan] Convert EVL loops to variable-length stepping after dissolution (PR #147222)

[Florian Hahn via llvm-commits]

================
@@ -2357,6 +2357,71 @@ bool VPlanTransforms::tryAddExplicitVectorLength(
   return true;
 }
 
+void VPlanTransforms::simplifyEVLIVs(VPlan &Plan) {
+  using namespace llvm::VPlanPatternMatch;
+  // Find EVL loop entries by locating VPEVLBasedIVPHIRecipe
+  // There should be only one EVL PHI in the entire plan
+  VPEVLBasedIVPHIRecipe *EVLPhi = nullptr;
+
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_shallow(Plan.getEntry())))
+    for (VPRecipeBase &R : VPBB->phis())
+      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
+        assert(!EVLPhi && "Found multiple EVL PHIs. Only one expected");
+        EVLPhi = PhiR;
+      }
+
+  // Early return if no EVL PHI is found
----------------
fhahn wrote:

```suggestion
  // Early return if no EVL PHI is found.
```

https://github.com/llvm/llvm-project/pull/147222