[VPlan] Use RPOT in CSE, fixing potential crash (#187548) A CSE crash is observed arising from outdated hash values unless we forbid replacements in successor phis in blocks that are not dominated by the def: the crash is observed when there is a block with CSE'able phis with CSE'able incoming values, with incoming values coming from a non-dominating block, under the condition that the block with the phis is visited before the non-dominating block. It is unfortunately impossible to write a test case showing a crash at present, but crashes do occur when attempting to CSE DerivedIV recipes. The root cause of the crash is visiting a non-dominated use before a def, and hence would be fixed by a reverse post-order traversal. Fixes #187499. Co-authored-by: Luke Lau <luke@igalia.com>
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