[dev.typeparams] cmd/compile: use dictionary to convert arguments of ==, != to interfaces

When comparing a value whose type is a type parameter to an interface,
we need to convert that type parameter to an interface using the dictionary
entries.

Change-Id: I409c9e36e376fe4ef8163407d0fd4e84496d5b65
Reviewed-on: https://go-review.googlesource.com/c/go/+/334150


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>