[SLP]Do not to try to revectorize previously vectorized phis in loops

No need to try to revectorize previously vectorized phis in loops, it leads to
a compile time blow-up.

Fixes #155998

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -11023,7 +11023,10 @@ BoUpSLP::ScalarsVectorizationLegality BoUpSLP::getScalarsVectorizationLegality(
       }
       SmallPtrSet<Value *, 8> Values(llvm::from_range, E->Scalars);
       if (all_of(VL, [&](Value *V) {
-            return isa<PoisonValue>(V) || Values.contains(V);
+            return isa<PoisonValue>(V) || Values.contains(V) ||
+                   (S.getOpcode() == Instruction::PHI && isa<PHINode>(V) &&
+                    LI->getLoopFor(S.getMainOp()->getParent()) &&
+                    isVectorized(V));
           })) {
         LLVM_DEBUG(dbgs() << "SLP: Gathering due to full overlap.\n");
         return ScalarsVectorizationLegality(S, /*IsLegal=*/false);