cmd/internal/gc: more int to bool cleanups

cmd/internal/gc: more int to bool cleanups

- make paramoutheap return a bool
- convert Node.Assigned to a bool
- convert Node.Captured to a bool
- convert Node.Byval to a bool
- convert Node.Dupok to a bool
- convert Node.Wrapper to a bool
- convert Node.Reslice to a bool

Change-Id: I5b57c019f936c31d53db4db14459fb2b0aa72305
Reviewed-on: https://go-review.googlesource.com/7030
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>