[dev.typeparams] cmd/compile: cleanup wrapper code for generics

Simple change - added some comments, but also removed some TODO comments
which are now done or no longer a question. Cleaned up the initial check
for generic methods.

The one remaining TODO that really needs to be done is generating
dictionary wrappers for any methods involving embedded fields. Given we
are not doing this, I think this would only affect if a struct with an
embedded field with methods was converted to an interface containing the
embedded method, and then the method was called via that interface.

Change-Id: I6a8a2885de33ad60b313c1899b659daef7550dfb
Reviewed-on: https://go-review.googlesource.com/c/go/+/340260


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>