[CaptureTracking] Avoid overly restrictive dominates check

CapturesBefore tracker has an overly restrictive dominates check when
the `BeforeHere` and the capture point are in different basic blocks.
All we need to check is that there is no path from the capture point
to `BeforeHere` (which is less stricter than the dominates check).
See added testcase in one of the users of CapturesBefore.

Reviewed-By: jdoerfert
Differential Revision: https://reviews.llvm.org/D90688