[dev.typeparams] cmd/compile: introduce OCONVIDATA op

This operation computes just the data field needed to put its argument
into an interface. Used by generics because we produce the type field
of an interface using dictionaries (instead of statically).

With this operation defined, we can now assert that shape types
are never marked as used in interfaces (the only previous use
was IDATA(CONVIFACE(t))).

Change-Id: Idb1eb5f3b238285cb99413d382599c0621b7681a
Reviewed-on: https://go-review.googlesource.com/c/go/+/337109


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