[X86] Fold VPERMV3(X,M,Y) -> VPERMV(CONCAT(X,Y),WIDEN(M)) iff the CONCAT is free (#122485) This extends the existing fold which concatenates X and Y if they are sequential subvectors extracted from the same source. By using combineConcatVectorOps we can recognise other patterns where X and Y can be concatenated for free (e.g. sequential loads, concatenating repeated instructions etc.), which allows the VPERMV3 fold to be a lot more aggressive. This required combineConcatVectorOps to be extended to fold the additional case of "concat(extract_subvector(x,lo), extract_subvector(x,hi)) -> extract_subvector(x)", similar to the original VPERMV3 fold where "x" was larger than the concat result type. This also exposes more cases where we have repeated vector/subvector loads if they have multiple uses - e.g. where we're loading a ymm and the lo/hi xmm pairs independently - in the past we've always considered this to be relatively benign, but I'm not certain if we should now do more to keep these from splitting?
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