[SCEV] Tweak the algorithm for figuring out if flags must apply to a SCEV [mostly-NFC]

Behavior wise, this patch should be mostly NFC.  The only behavior difference known is that on the isSCEVExprNeverPoison path we'll consider a bound imposed by the SCEVable operands (if any).

Algorithmically, it's an invert of the existing code.  Previously, we checked for each operand if we could find a bound, then checked for must-execute given that bound.  With the patch, we use dominance to refine the innermost bound, then check must execute once.  The interesting case is when we have multiple unknowns within a single basic block.  While both dominance and must-execute are worst-case linear walks within the block, only dominance is cached.  As such, refining based on dominance should be more efficient.

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