[DA] Fix function `DependenceInfo::isKnownPredicate`

[sjoerdmeijer]

This is extracted from #160924 and little bit of a placeholder (more details will follow), but this is an attempt to document all know DA issues:

The complaint about function DependenceInfo::isKnownPredicate was the following:

Removing zext can change the result (consider the value of zext(x) when x is a negative value). Probably we are not allowed to purge zext in general. Also, it's unclear what happens if getMinusSCEV wraps.

I will add this to the meta issue #161805 that lists all DA issues.

[sjoerdmeijer]

CC: @kasuga-fj