SCEV isKnownPredicate may crash if the expressions are involved with
different loops. To verify if two loops have the same iteration space,
we do not need to use the SCEV apis, and it can be done by the equality
check.
Moreover, no pass (not even loop fusion) requires to check SameSD levels
for more than one level. In this patch, we limit the analysis of SameSD
levels to only one level after the common levels.
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