[BasicAA][LAA] Don't use same-block phis in cross iteration mode

In 4de3184 we exposed BasicAA's
cross-iteration mode for use in LAA, so we can handle selects
with equal conditions correctly (where the select condition is
not actually equal across iterations).

However, if we replace the selects with equivalent phis, the
issue still exists. In the phi case, we effectively still have
an assumption that the condition(s) that control which phi arg
is used will be the same across iterations. Fix this by disabling
this case in cross-iteration mode.

Author: nikic

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -1447,9 +1447,10 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
     return AliasResult::NoAlias;
   // If the values are PHIs in the same block, we can do a more precise
   // as well as efficient check: just check for aliases between the values
-  // on corresponding edges.
+  // on corresponding edges. Don't do this if we are analyzing across
+  // iterations, as we may pick a different phi entry in different iterations.
   if (const PHINode *PN2 = dyn_cast<PHINode>(V2))
-    if (PN2->getParent() == PN->getParent()) {
+    if (PN2->getParent() == PN->getParent() && !AAQI.MayBeCrossIteration) {
       std::optional<AliasResult> Alias;
       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
         AliasResult ThisAlias = AAQI.AAR.alias(

llvm/test/Analysis/LoopAccessAnalysis/select-dependence.ll

@@ -44,8 +44,13 @@ exit:
 define void @test_phi(ptr noalias %x, ptr noalias %y, ptr noalias %z) {
 ; CHECK-LABEL: 'test_phi'
 ; CHECK-NEXT:    loop:
-; CHECK-NEXT:      Memory dependences are safe
+; CHECK-NEXT:      Report: unsafe dependent memory operations in loop. Use #pragma clang loop distribute(enable) to allow loop distribution to attempt to isolate the offending operations into a separate loop
+; CHECK-NEXT:  Unsafe indirect dependence.
 ; CHECK-NEXT:      Dependences:
+; CHECK-NEXT:        IndirectUnsafe:
+; CHECK-NEXT:            %load = load double, ptr %gep.sel, align 8 ->
+; CHECK-NEXT:            store double %load, ptr %gep.sel2, align 8
+; CHECK-EMPTY:
 ; CHECK-NEXT:      Run-time memory checks:
 ; CHECK-NEXT:      Grouped accesses:
 ; CHECK-EMPTY: