Reapply [SCEV] Replace IsAvailableOnEntry with block disposition

Reapply [SCEV] Replace IsAvailableOnEntry with block disposition

This exposed an issue in SCEVExpander/LCSSA, which has been fixed
in D150681.

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As far as I understand, the IsAvailableOnEntry() function basically
implements the same functionality as the properlyDominates() block
disposition. The primary difference (apart from a weaker
implementation) seems to be in this comment at the top:

    // Checks if the SCEV S is available at BB.  S is considered available at BB
    // if S can be materialized at BB without introducing a fault.

However, I don't really understand why there would be such a
requirement. It's my understanding that SCEV explicitly does not
care about trapping udiv instructions itself, and it's the job of
SCEVExpander's isSafeToExpand() to make sure these don't get
expanded if they may trap.

Differential Revision: https://reviews.llvm.org/D149344