[SCEV] Use power of two facts involving vscale when inferring wrap flags (#101380)

SCEV has logic for inferring wrap flags on AddRecs which are known to
control an exit based on whether the step is a power of two. This logic
only considered constants, and thus did not trigger for steps such as (4
x vscale) which are common in scalably vectorized loops.

The net effect is that we were very sensative to the preservation of
nsw/nuw flags on such IVs, and could not infer trip counts if they got
lost for any reason.

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Co-authored-by: Nikita Popov <github@npopov.com>

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