Introduce a try_ntype function which doesn't return an error upon
not finding a type. Use it instead of having separate repeated
token checks. Simpler, less code, and more efficient.

Change-Id: I81e482158b71901eb179470269349688636aa0ba
Reviewed-on: https://go-review.googlesource.com/17157Reviewed-by: Chris Manghane <cmang@golang.org>