[Attributor] Fix an issue that an access is skipped by mistake

When we check if an access can be skipped, there is a case that an
inter-procedural interference access exists after a dominant write. Currently we
rely on `AAInterFnReachability` to tell if the access can be reachable. If it is
not, we can safely skip the access. However, it is based on an assumption that
the AA exists. It is possible that the AA doesn't exist. In this case, we can't
safely assume the acess can be skipped because we have to assume the access can
reach. This can happen when `AAInterFnReachability` is not in the allowed AA
list when creating the attributor, such as AMDGPUAttributor.

Author: shiltian

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

@@ -1331,7 +1331,7 @@ struct AAPointerInfoImpl
         // itself either.
         bool Inserted = ExclusionSet.insert(&I).second;
 
-        if (!FnReachabilityAA ||
+        if (FnReachabilityAA &&
             !FnReachabilityAA->instructionCanReach(
                 A, *LeastDominatingWriteInst,
                 *Acc.getRemoteInst()->getFunction(), &ExclusionSet))