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This patch extends the SelectionDAG's ability to constant-fold vector arithmetic to include support for SPLAT_VECTOR. This is not only for scalable-vector types but also for fixed-length vector types, which helps Hexagon in a couple of cases. The original RISC-V test case was in fact an infinite DAGCombine loop. The pattern `and (truncate v1), (truncate v2)` can be combined to `truncate (and v1, v2)` but the truncate can similarly be combined back to `truncate (and v1, v2)` (but, crucially, only when one of `v1` or `v2` is a constant vector). It wasn't exposed in on fixed-length types because a TRUNCATE of a constant BUILD_VECTOR was folded into the BUILD_VECTOR itself, whereas this did not happen for the equivalent (scalable-vector) SPLAT_VECTOR. Reviewed By: RKSimon, craig.topper Differential Revision: https://reviews.llvm.org/D103246
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