[Attributor][FIX] checkForAllInstructions, correctly handle declered functions

[kuter]

Summary

checkForAllInstructions was not handling declared functions correctly.
It should have been returning false when it gets called on a declared function.

The patch also fixes a test case for AAFunctionReachability for it to be able
to pass after the changes to the checkForAllinstructions.

[jdoerfert]

LG. Call them declarations int the commit message and subject, not declared function (a definition is also a declaration and declares a function but people will know).

llvm/lib/Transforms/IPO/Attributor.cpp

bool Attributor::checkForAllInstructions(function_ref<bool(Instruction &)> Pred,
                                          bool CheckPotentiallyDead) {
 
   const IRPosition &IRP = QueryingAA.getIRPosition();
   // Since we need to provide instructions we have to have an exact definition.
   const Function *AssociatedFunction = IRP.getAssociatedFunction();
   if (!AssociatedFunction)
     return false;
 
+  if (AssociatedFunction->isDeclaration())
+    return false;
+
   // TODO: use the function scope once we have call site AAReturnedValues.
   const IRPosition &QueryIRP = IRPosition::function(*AssociatedFunction);
   const auto *LivenessAA =
       (CheckBBLivenessOnly || CheckPotentiallyDead)
           ? nullptr
           : &(getAAFor<AAIsDead>(QueryingAA, QueryIRP, DepClassTy::NONE));
 
   auto &OpcodeInstMap =

llvm/unittests/Transforms/IPO/AttributorTest.cpp

TEST_F(AttributorTestBase, TestCast) {
   const auto *SSucc = dyn_cast<AAIsDead>(AA);
 
   ASSERT_EQ(SFail, nullptr);
   ASSERT_TRUE(SSucc);
 }
 
 TEST_F(AttributorTestBase, AAReachabilityTest) {
   const char *ModuleString = R"(
-    declare void @func4()
-    declare void @func3()
+    @x = global i32 0
+    define void @func4() {
+      store i32 0, i32* @x
+      ret void
+    }
+
+    define void @func3() {
+      store i32 0, i32* @x
+      ret void
+    }
 
     define void @func2() {
     entry:
       call void @func3()
       ret void
     }
 
     define void @func1() {