[BasicAA] Handle phi with itself as incoming value

We can skip such incoming values. This was already done by
PhiValues if present, but we can also do this without the
additional analysis.

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1360,6 +1360,10 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
     SmallPtrSet<Value *, 4> UniqueSrc;
     Value *OnePhi = nullptr;
     for (Value *PV1 : PN->incoming_values()) {
+      // Skip the phi itself being the incoming value.
+      if (PV1 == PN)
+        continue;
+
       if (isa<PHINode>(PV1)) {
         if (OnePhi && OnePhi != PV1) {
           // To control potential compile time explosion, we choose to be

llvm/test/Analysis/BasicAA/recphi.ll

@@ -413,8 +413,7 @@ exit:
   ret void
 }
 
-; NO-PHI-VALUES: MayAlias: i8* %a, i8* %phi
-; PHI-VALUES: MustAlias: i8* %a, i8* %phi
+; CHECK: MustAlias: i8* %a, i8* %phi
 define void @phi_contains_self() {
 entry:
   %a = alloca i32