[mypyc] Support undefined locals with native int types (#13616)
There's no reserved value we can use to track if a local variable with
a native int type hasn't been initialized yet. Instead, use bitmaps to
track whether a local is defined, but only if the local has a native
int type and can be undefined.
Work on https://github.com/mypyc/mypyc/issues/837.
