Separate out Check for Branch Always Taken

[DavePearce]

At the moment, the branch always taken check is embedded within FlowTypeCheck. This actually causes problems because it results in errors being raise prematurely. For example, consider this:

method main():
    int|null x = null

    while x is null:
        x = 1

This generates the error even though its not actually true. The problem is that we need to really get the full type for x at that point before reporting the error.