[LoopVectorize] Make needsExtract notice scalarized instructions (#119720) 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.
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