go/types, types2: shorter list for 2nd phase of function type inference

In the 2nd phase of function argument type inference we only
consider parameters with types that are single type parameters.
Thus there is no need to collect anything else in the first
phase.

This matches the algorithm description in the forthcoming spec
more closely.

Change-Id: Ie5c29f30ff43b1e37d719ecbe1688b50ed2177f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/381554


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>