[LoopVectorize] Make needsExtract notice scalarized instructions

LoopVectorizationCostModel::needsExtract should recognise instructions
that have been widened by scalarizing as scalar instructions, and thus
not needing an extract when used by later scalarized instructions.

This fixes an incorrect cost calculation in computePredInstDiscount,
where we are adding a scalarization overhead cost when we shouldn't,
though I haven't come up with a test case where it makes a difference.
It will make a difference when the cost model switches to using the
cost kind TCK_CodeSize for optsize, as not doing this causes the test
LoopVectorize/X86/small-size.ll to get worse.

Author: john-brawn-arm

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1741,7 +1741,8 @@ class LoopVectorizationCostModel {
   bool needsExtract(Value *V, ElementCount VF) const {
     Instruction *I = dyn_cast<Instruction>(V);
     if (VF.isScalar() || !I || !TheLoop->contains(I) ||
-        TheLoop->isLoopInvariant(I))
+        TheLoop->isLoopInvariant(I) ||
+        getWideningDecision(I, VF) == CM_Scalarize)
       return false;
 
     // Assume we can vectorize V (and hence we need extraction) if the

llvm/test/Transforms/LoopVectorize/AArch64/interleaved_cost.ll

@@ -170,8 +170,8 @@ entry:
 ; VF_2-LABEL: Checking a loop in 'i64_factor_8'
 ; VF_2:         Found an estimated cost of 8 for VF 2 For instruction: %tmp2 = load i64, ptr %tmp0, align 8
 ; VF_2-NEXT:    Found an estimated cost of 8 for VF 2 For instruction: %tmp3 = load i64, ptr %tmp1, align 8
-; VF_2-NEXT:    Found an estimated cost of 12 for VF 2 For instruction: store i64 %tmp2, ptr %tmp0, align 8
-; VF_2-NEXT:    Found an estimated cost of 12 for VF 2 For instruction: store i64 %tmp3, ptr %tmp1, align 8
+; VF_2-NEXT:    Found an estimated cost of 8 for VF 2 For instruction: store i64 %tmp2, ptr %tmp0, align 8
+; VF_2-NEXT:    Found an estimated cost of 8 for VF 2 For instruction: store i64 %tmp3, ptr %tmp1, align 8
 for.body:
   %i = phi i64 [ 0, %entry ], [ %i.next, %for.body ]
   %tmp0 = getelementptr inbounds %i64.8, ptr %data, i64 %i, i32 2