[Polly] Assumptions used to derive domain must not be pruned by that domain (#190436)

The code that emits the conditions for whether a statement is executed
by checking whether we are in the statement's domain may apply
assumptions (such as an integer truncation being reversible). Later code
then assumes that these assumptions are only relevent for then the
statement is executed, but actually it is used for determining whether
it is executed.

Break this circular reasoning by introducing an `IsInsideDomain` flag
that can be set when the domain has not been verified yet.

Fixes #190128

Thanks to @thapgua for the bug report

GitOrigin-RevId: 948a64720b1ec018e13b4bb8250482f9615d7856