Data flow doesn't track per-branch values correctly

[vitek-karas]

			string str = GetWithPublicMethods ();
			if (String.Empty.Length == 0) {
				str = GetWithPublicFields (); // dataflow will merge this with the value from the previous basic block
				RequirePublicFields (str); // produces a warning
			}

In the above sample, at the point of calling RequirePublicFields the str local is guaranteed to have the value coming only from GetWithPublicFields(), so it should not warn. But linker doesn't correctly track the values in this case.

There is an existing test case which covers this, but we don't have an issue for it:

linker/test/Mono.Linker.Tests.Cases/DataFlow/LocalDataFlow.cs

Lines 224 to 232 in 5fca8b8

	[UnrecognizedReflectionAccessPattern (typeof (LocalDataFlow), nameof (RequirePublicFields), new Type[] { typeof (string) }, messageCode: "IL2072")]
	public static void TestBranchIf ()
	{
	string str = GetWithPublicMethods ();
	if (String.Empty.Length == 0) {
	str = GetWithPublicFields (); // dataflow will merge this with the value from the previous basic block
	RequirePublicFields (str); // produces a warning
	}
	}