Automerge: [SLP]Recalculate number of parts when requesting number of elements based on original scalars size

Need to recalculate number of parts, since gathered scalar size might be changed
during building the buildvector shuffles.

Fixes #125259

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -15094,7 +15094,9 @@ ResTy BoUpSLP::processBuildVector(const TreeEntry *E, Type *ScalarTy,
       }
     }
     if (!GatherShuffles.empty()) {
-      unsigned SliceSize = getPartNumElems(E->Scalars.size(), NumParts);
+      unsigned SliceSize =
+          getPartNumElems(E->Scalars.size(),
+                          ::getNumberOfParts(*TTI, VecTy, E->Scalars.size()));
       SmallVector<int> VecMask(Mask.size(), PoisonMaskElem);
       for (const auto [I, TEs] : enumerate(Entries)) {
         if (TEs.empty()) {

llvm/test/Transforms/SLPVectorizer/X86/gathered-shuffle-resized.ll

@@ -18,9 +18,9 @@ define ptr @test(ptr %0, ptr %args_gep) {
 ; CHECK-NEXT:    [[TMP8:%.*]] = load i32, ptr [[TMP7]], align 4, !noalias [[META0:![0-9]+]]
 ; CHECK-NEXT:    [[TMP9:%.*]] = load <2 x i32>, ptr [[TMP6]], align 8, !noalias [[META0]]
 ; CHECK-NEXT:    [[TMP10:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <4 x i32> <i32 0, i32 1, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP11:%.*]] = shufflevector <2 x i32> [[TMP9]], <2 x i32> poison, <16 x i32> poison
+; CHECK-NEXT:    [[TMP11:%.*]] = shufflevector <2 x i32> [[TMP9]], <2 x i32> poison, <16 x i32> <i32 poison, i32 poison, i32 0, i32 1, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
 ; CHECK-NEXT:    [[TMP12:%.*]] = shufflevector <4 x i32> [[TMP10]], <4 x i32> poison, <16 x i32> <i32 0, i32 1, i32 2, i32 3, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP13:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> [[TMP9]], <16 x i32> <i32 0, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison, i32 poison>
+; CHECK-NEXT:    [[TMP13:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> [[TMP9]], <16 x i32> <i32 0, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 2, i32 2, i32 2, i32 2, i32 2, i32 2, i32 3, i32 3, i32 3>
 ; CHECK-NEXT:    [[TMP14:%.*]] = icmp eq <16 x i32> [[TMP13]], zeroinitializer
 ; CHECK-NEXT:    [[TMP15:%.*]] = zext <16 x i1> [[TMP14]] to <16 x i8>
 ; CHECK-NEXT:    store <16 x i8> [[TMP15]], ptr [[TMP5]], align 1