cmd/compile: compute number of arguments correctly
The outCount value includes a flag bit for dotdotdot.
If we have this count incorrect, then the offset for the
methodset *rtype are in the wrong place.
Fixes #14783
Change-Id: If5acb16af08d4ffe36c8c9ee389c32f2712ce757
Reviewed-on: https://go-review.googlesource.com/20566 Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>