[unittests] add callbr AliasAnalysisTest case

[nickdesaulniers]

Summary

Demonstrate that AA can determine that callbr inputs don't alias with
globals. See also: https://reviews.llvm.org/D144927

To run test:
$ pushd llvm/build; ninja AnalysisTests; popd
$ ./llvm/build/unittests/Analysis/AnalysisTests \

--gtest_filter=AliasAnalysisTest.CallBr

llvm/unittests/Analysis/AliasAnalysisTest.cpp

TEST_F(AliasAnalysisTest, PartialAliasOffsetSign) {
   auto AR = AA.alias(Loc1, Loc2);
   EXPECT_EQ(AR, AliasResult::PartialAlias);
   EXPECT_EQ(1, AR.getOffset());
 
   AR = AA.alias(Loc2, Loc1);
   EXPECT_EQ(AR, AliasResult::PartialAlias);
   EXPECT_EQ(-1, AR.getOffset());
 }
+TEST_F(AliasAnalysisTest, CallBr) {
+  LLVMContext C;
+  SMDiagnostic Err;
+  std::unique_ptr<Module> M = parseAssemblyString(R"(
+@y = global i32 0
+@x = global i32 0
+define i32 @foo4() {
+entry:
+  callbr void asm "", "=*m,!i"(ptr elementtype(i32) @x)
+  to label %a [label %b]
+
+a:                                  ; preds = %entry
+  %z = load i32, ptr @y
+  ret i32 %z
+
+b:                                        ; preds = %entry
+  %w = load i32, ptr @y
+  ret i32 %w
+}
+)", Err, C);
+  if (!M)
+    Err.print("CallBr", errs());
+
+  Function *F = M->getFunction("foo4");
+  const auto Loc1 = MemoryLocation::get(getInstructionByName(*F, "z"));
+  const auto Loc2 = MemoryLocation::get(getInstructionByName(*F, "w"));
+  auto &AA = getAAResults(*F);
+  auto AR = AA.alias(Loc1, Loc2);
+  EXPECT_EQ(AR, AliasResult::MustAlias);
+
+  CallBrInst *CBR;
+  for (BasicBlock &BB : *F)
+    for (Instruction &I : BB)
+      if (isa<CallBrInst>(I))
+        CBR = cast<CallBrInst>(&I);
+
+  const auto Loc3 = MemoryLocation::getForArgument(CBR, 0, nullptr);
+  AR = AA.alias(Loc1, Loc3);
+  EXPECT_EQ(AR, AliasResult::NoAlias);
+  AR = AA.alias(Loc2, Loc3);
+  EXPECT_EQ(AR, AliasResult::NoAlias);
+}
 class AAPassInfraTest : public testing::Test {
 protected:
   LLVMContext C;
   SMDiagnostic Err;
   std::unique_ptr<Module> M;
 
 public:
   AAPassInfraTest()