cmd/compile/internal/types2: reduce number of delayed functions (optimization)

Rather than create and delay execution of a closure for each type parameter
in a type parameter list, just create one per type parameter list.

While at it, inline the small amount of code for getting the type constraint
and remove the respective function.

Change-Id: I49a00ff0a7b7e43eb53992dd7dbfac25ff23b42c
Reviewed-on: https://go-review.googlesource.com/c/go/+/348018


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