Minify loop labels while we're minifying local names.

This uses the minifyLocals pass as an opportunity to also minify loop labels.  AFAICT it's safe for these to overlap with variable declarations etc, so it treats them as a totally separate namespace.

The improvement in code size is admittedly pretty tiny.  For pypy.js I see a saving of 8KB in gzipped code size, from 3548K down to 3540K.  On the other hand, it comes at minimal cost since we're doing the traversal anyway, so maybe it's still worthwhile.