We clear locals when we know their values might change (ex. when performing a method call). However previously values on the stack which were originally pushed from a local would still point back to that local. With this commit, when clearing locals, we'll now iterate over the mappings of the stack and copy the known type from the local to the stack mapping, removing the association to the local. This should mean both that we'll retain any information we already know about the local type, and that if a local is modified we won't incorrectly infer it's new type from the existing value on the stack.