[PATCH] PR15413: Assertion "SE.DT->dominates(Ret, BIP)" fails.

[Andrew Trick]

Please review this patch. It addresses a fundamental problem with SSA update. There have been attempts to fix and work around it, but they only hide the problem. I believe this patch moves in the right direction.

We still need to do something about the interaction between LCSSA and SCEV (LCSSA should be removed), and metadata operands are still not updated correctly (SSA updater needs to handle handles).

In this particular case, both LCSSA and SSA updater were incorrectly updating SCEV such that SCEVExpander would blow up later in the vectorizer. This needed to be fixed without regressing on previous efforts to avoid total SCEV invalidation and preserve correct behavior when updating other ValueHandles as seen in PR12901.

-Andy
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