[AliasSetTracker] Make AST smarter about assume intrinsics that don't actually affect memory.

[mcrosier]

Summary

Reapply r285191, but only include assume intrinsics, per Hal's suggestion. Also reference the associated PR.

Chad

[hfinkel]

LGTM, thanks!

lib/Analysis/AliasSetTracker.cpp

AliasSet &ASDst =
       addPointer(MTI->getRawDest(), Len, AAInfo, AliasSet::ModAccess);
   if (MTI->isVolatile())
     ASDst.setVolatile();
 }
 
 void AliasSetTracker::addUnknown(Instruction *Inst) {
   if (isa<DbgInfoIntrinsic>(Inst))
     return; // Ignore DbgInfo Intrinsics.
+
+  if (auto *II = dyn_cast<IntrinsicInst>(Inst)) {
+    // These intrinsics will show up as affecting memory, but they are just
+    // markers.
+    switch (II->getIntrinsicID()) {
+    default:
+      break;
+      // FIXME: Add lifetime/invariant intrinsics (See: PR30807).
+    case Intrinsic::assume:
+      return;
+    }
+  }
   if (!Inst->mayReadOrWriteMemory())
     return; // doesn't alias anything
 
   AliasSet *AS = findAliasSetForUnknownInst(Inst);
   if (AS) {
     AS->addUnknownInst(Inst, AA);
     return;
   }

test/Analysis/AliasSet/intrinsics.ll

+; RUN: opt -basicaa -print-alias-sets -S -o - < %s 2>&1 | FileCheck %s
+
+; CHECK: Alias sets for function 'test1':
+; CHECK: Alias Set Tracker: 2 alias sets for 2 pointer values.
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %a, 1)
+; CHECK-NOT: 1 Unknown instruction
+; CHECK:   AliasSet[0x{{[0-9a-f]+}}, 1] must alias, Mod       Pointers: (i8* %b, 1)
+define void @test1(i32 %c) {
+entry:
+  %a = alloca i8, align 1
+  %b = alloca i8, align 1
+  store i8 1, i8* %a, align 1
+  %cond1 = icmp ne i32 %c, 0
+  call void @llvm.assume(i1 %cond1)
+  store i8 1, i8* %b, align 1
+  ret void
+}
+
+declare void @llvm.assume(i1)