[BasicAA] Only add visited phi blocks temporarily

Visited phi blocks only need to be added for the duration of the
recursive alias queries, they should not leak into following code.

Once again, while this also improves analysis precision, this is
mainly intended to clarify the applicability scope of VisitedPhiBBs.

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -14,6 +14,7 @@
 
 #include "llvm/Analysis/BasicAliasAnalysis.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
@@ -814,7 +815,7 @@ AliasResult BasicAAResult::alias(const MemoryLocation &LocA,
   AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.AATags, LocB.Ptr,
                                  LocB.Size, LocB.AATags, AAQI);
 
-  VisitedPhiBBs.clear();
+  assert(VisitedPhiBBs.empty());
   return Alias;
 }
 
@@ -1654,7 +1655,11 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
   // In the recursive alias queries below, we may compare values from two
   // different loop iterations. Keep track of visited phi blocks, which will
   // be used when determining value equivalence.
-  VisitedPhiBBs.insert(PN->getParent());
+  auto Pair = VisitedPhiBBs.insert(PN->getParent());
+  auto _ = make_scope_exit([&]() {
+    if (Pair.second)
+      VisitedPhiBBs.erase(PN->getParent());
+  });
 
   AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo, V1Srcs[0], PNSize,
                                  PNAAInfo, AAQI, UnderV2);

llvm/test/Analysis/BasicAA/phi-aa.ll

@@ -116,7 +116,7 @@ end:
 }
 
 ; CHECK-LABEL: phi_and_select
-; CHECK: MayAlias: i32* %p, i32* %s
+; CHECK: MustAlias: i32* %p, i32* %s
 define void @phi_and_select(i1 %c, i1 %c2, i32* %x, i32* %y) {
 entry:
   br i1 %c, label %true, label %false