[dev.typeparams] cmd/compile: use dictionary entries for more conversion cases

This CL handles I(x) where I is an interface type and x has
typeparam type.

Change-Id: Ib99de2b741d588947f5e0164255f6365e98acd8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/326189


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>