[Lint] Skip null args when checking noalias

Do not emit a warning if there are two null noalias arguments,
as they cannot be dereferenced anyway.

This is a common pattern for `@.omp_outlined`, which has some
optional noalias arguments.

Author: nikic

llvm/lib/Analysis/Lint.cpp

@@ -244,7 +244,8 @@ void Lint::visitCallBase(CallBase &I) {
             // dereferenced anyway.
             if (I.doesNotAccessMemory(ArgNo))
               continue;
-            if (AI != BI && (*BI)->getType()->isPointerTy()) {
+            if (AI != BI && (*BI)->getType()->isPointerTy() &&
+                !isa<ConstantPointerNull>(*BI)) {
               AliasResult Result = AA->alias(*AI, *BI);
               Check(Result != AliasResult::MustAlias &&
                         Result != AliasResult::PartialAlias,

llvm/test/Analysis/Lint/noalias-null.ll

@@ -0,0 +1,14 @@
+; RUN: opt < %s -passes=lint -disable-output 2>&1 | FileCheck --allow-empty %s
+
+declare void @foo(ptr noalias, ptr noalias)
+
+define void @test() {
+entry:
+  call void @foo(ptr null, ptr null)
+  ret void
+}
+
+; Lint should not complain about passing null to both arguments if they are
+; null, since noalias only applies if the argument is written to, which is not
+; possible for a null pointer.
+; CHECK-NOT: Unusual: noalias argument aliases another argument