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DAGCombine's `mergeStoresOfConstantsOrVecElts` optimization is told whether it's to use vector types and also whether it's to issue a truncating store. However, the truncating store code path assumes a scalar integer `ConstantSDNode`, and when using vector types it creates either a `BUILD_VECTOR` or `CONCAT_VECTORS` to store: neither of which is a constant. The `riscv64` target is able to expose a crash here because it switches on both code paths at the same time. The `f32` is stored as `i32` which must be promoted to `i64`, necessitating a truncating store. It also decides later that it prefers a vector store of `v2f32`. While vector truncating stores are legal, this combine is not able to emit them. We also don't have a test case. This patch adds an assert to catch this case more gracefully, and updates one of the caller functions to the function to turn off the use of truncating stores when preferring vectors. Reviewed By: craig.topper Differential Revision: https://reviews.llvm.org/D103173
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