[NFC]{MLInliner] Point out the tests' model dependencies

llvm/lib/Analysis/models/inliner/README.txt

@@ -0,0 +1,3 @@
+Reference model for inliner -Oz decision policy.
+Note that, currently, this model is also referenced by test/Transforms/Inline/ML
+tests - if replacing it, check those tests, too.

llvm/unittests/Analysis/TFUtilsTest.cpp

@@ -21,6 +21,8 @@ using namespace llvm;
 
 extern const char *TestMainArgv0;
 
+// NOTE! This test model is currently also used by test/Transforms/Inline/ML tests
+//- relevant if updating this model.
 static std::string getModelPath() {
   SmallString<128> InputsDir = unittest::getInputFileDirectory(TestMainArgv0);
   llvm::sys::path::append(InputsDir, "ir2native_x86_64_model");