[ScopBuilder] Do not verify unfeasible SCoPs.

SCoPs with unfeasible runtime context are thrown away and therefore
do not need their uses verified.

The added test case requires a complexity limit to exceed.
Normally, error statements are removed from the SCoP and for that
reason are skipped during the verification. If there is a unfeasible
runtime context (here: because of the complexity limit being reached),
the removal of error statements and other SCoP construction steps are
skipped to not waste time. Error statements are not modeled in SCoPs
and therefore have no requirements on whether the scalars used in
them are available.

llvm-svn: 302234