[SelectionDAG] Fold VECREDUCE_ADD of a constant BUILD_VECTOR (#207560) This PR implements constant folding for ISD::VECREDUCE_ADD when the input is a BUILD_VECTOR of integer constants. The fold computes the sum at compile time using APInt arithmetic and returns a folded constant scalar, instead of emitting real vector materialization and reduction instructions for a value that's already known. Folding is skipped if any element is undef or opaque, since the result can't be assumed at compile time in that case. Fixes #206743
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