[DAGCombine] Fix an incorrect folding of extract_subvector (#153709) Reported from https://github.com/llvm/llvm-project/pull/153393#issuecomment-3189898813 During DAGCombine, an intermediate extract_subvector sequence was generated: ``` t8: v9i16 = extract_subvector t3, Constant:i64<9> t24: v8i16 = extract_subvector t8, Constant:i64<0> ``` And one of the DAGCombine rule which turns `(extract_subvector (extract_subvector X, C), 0)` into `(extract_subvector X, C)` kicked in and turn that into `v8i16 = extract_subvector t3, Constant:i64<9>`. But it forgot to check if the extracted index is a multiple of the minimum vector length of the result type, hence the crash. This patch fixes this by adding an additional check.
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