[X86] combineX86ShuffleChain - don't combine to VPERM2W/VPERM2B from just any single variable mask (#127914) Despite them being more expensive than other variable mask shuffles, we were combining shuffle chains to VPERM2W/VPERM2B if any shuffle in the chain was a variable shuffle - including very cheap shuffles like PSHUFB or AND mask patterns. This patch adjusts the BWI VPERMV3 threshold - it still always permits the merge if the chain (of 2 or more shuffles) contains any X86ISD::VPERMV/VPERMV3 shuffles (including DQ variants), but otherwise only reduces the depth threshold based off the number of other variable shuffles we'd fold away.
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