[dev.typeparams] cmd/compile: change method instantiations back to being functions

Change all instantiated methods to being functions again. We found that
this is easier for adding the dictionary argument consistently. A method
wrapper will usually be added around the instantiation call, so that
eliminate the inconsistency in the type of the top-level method and the
the associated function node type.

Change-Id: I9034a0c5cc901e7a89e60756bff574c1346adbc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/321609


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