swift234

[rcid] Teach RCIdentityAnalaysis::getRCUses() how to ignore certain uses that are inert from its perspective.

This includes:

1. Extract instructions which extracts a trivial part of an aggregate that has
one RCIdentity.
2. Instructions which take a pointer out of ARC's control by converting it to a
trivial type. This is safe to do since we can assume that the object that is
convered is alive when the conversion happens. So assuming that we can
conservatively find all RC users, we will have at least one RC user that
post dominates the use (since otherwise we would be touching a dangling
pointer). We leave it to the user of the pass to determine what is safe to do
with this information. Potentially in the future it might make sense to return
this information as well so that a user can use that information directly.

rdar://20305817

Swift SVN r26583