cmd/compile: delay all transforms for generic funcs/methods

This change cleans up the code, by just delaying all transforms on
generic function methods/functions until stenciling time. That way, we
don't have extra code to decide whether to delay, or an extra value for
the typecheck flag. We are already doing all possible transforms at
stencil time anyway, so no changes to the stenciling code.

transform.go includes a change for one case where we check for shape
rather than tparam, now that we only apply transforms to stenciled
functions, not generic functions. This change is to allow CONVIFACE node
to be correctly inserted (needed for dictionaries), even with this
strange code that doesn't add the CONVIFACE node if the concrete type is
NOT huge...

Change-Id: I5f1e71fab11b53385902074915b3ad85f8e753fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/350736


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