[LAA] Include IndirectUnsafe in ::isPossiblyBackward.

Summary:
Similarly to Unknown, IndirectUnsafe should also be considered possibly
backward, as it may be a backwards dependency e.g. via loading
different base pointers.

This also brings isPossiblyBackward in line with
Dependence::isSafeForVectorization. At the moment this can't be tested,
as it is not possible to write a test with an AddRec that is based on a
loop varying value. But this may change in the future and may cause
mis-compiles in the future.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60251685

Author: fhahn

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -1730,7 +1730,7 @@ bool MemoryDepChecker::Dependence::isBackward() const {
 }
 
 bool MemoryDepChecker::Dependence::isPossiblyBackward() const {
-  return isBackward() || Type == Unknown;
+  return isBackward() || Type == Unknown || Type == IndirectUnsafe;
 }
 
 bool MemoryDepChecker::Dependence::isForward() const {