[llvm] [VPlan] Use ResumePhi to create reduction resume phis. (PR #110004)

[via llvm-commits]

================
@@ -7561,67 +7561,53 @@ static void addRuntimeUnrollDisableMetaData(Loop *L) {
   }
 }
 
-// Check if \p RedResult is a ComputeReductionResult instruction, and if it is
-// create a merge phi node for it.
-static void createAndCollectMergePhiForReduction(
-    VPInstruction *RedResult,
-    VPTransformState &State, Loop *OrigLoop, BasicBlock *LoopMiddleBlock,
-    bool VectorizingEpilogue) {
+// If \p R is a ComputeReductionResult when vectorizing the epilog loop,
+// update the reduction's scalar PHI node by adding the incoming value from the
+// main vector loop.
+static void updateMergePhiForReductionForEpilogueVectorization(
----------------
ayalz wrote:

```suggestion
static void fixReductionScalarResumeWhenVectorizingEpilog(
```
sounds better?
This case cannot be handled by recipes alone as it involves passing a value between recipes of distinct VPlans, hence requires "fixing".
There are several "Merge phis" involved, here we're dealing with the one in the scalar preheader feeding the value for the scalar loop to resume from. 
When executing the VPlan of the epilog loop, this phi considered 2 values (ignorant of the main loop) - the one coming from the epilog loop via its middle block, and the original initial value for bypassing from entry to scalar loop. But there is a 3rd value to consider - computed by the vectorized main loop and bypassing the epilog loop. This value was already fed into the "scalar preheader" (actually vector-epilog-tripcount-check block) when executing the VPlan of the main loop (which needs no fixing). We now look for this value and feed it into the resumePhi of the scalar preheader, corresponding to the edge from this block to the scalar preheader.

https://github.com/llvm/llvm-project/pull/110004