[dev.typeparams] cmd/compile: make type conversions by type parameters work

When doing a type conversion using a type param, delay the
transformation to OCONV/OCONVNOP until stenciling, since the nodes
created depend on the actual type.

Re-enable the fact.go test.

Change-Id: I3d5861aab3dd0e781d767f67435afaf951dfe451
Reviewed-on: https://go-review.googlesource.com/c/go/+/290752


Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>