[ScopDetection] Use addUnknown for calls in the AliasSetTracker.

The general-purpose add() now sometimes adds unexpected loop-variant
pointers to the AliasSetTracker, so certain loops would be rejected with
-polly-allow-modref-calls.  Use addUnknown() instead, which has the old
behavior.

I'm not completely convinced the resulting behavior is actually
correct: ScopDetection::isValidAccess seems to mostly ignore
"unknown" instructions in the AliasSetTracker.  But it's not any worse
than what was happening before.

Committing without pre-commit review to unbreak the buildbots; the
following tests were failing:

test/ScopInfo/mod_ref_access_pointee_arguments.ll
test/ScopInfo/mod_ref_read_pointee_arguments.ll
test/ScopInfo/multidim_2d_with_modref_call_2.ll