[dev.typeparams] cmd/compile: convert generic values to interface type using dictionary

When converting a variable of generic type to an interface, use the
entry in the dictionary for the type field instead of using the
compile-time type (which we only have when fully stenciling).

Note: this isn't all the conversions. Conversions often get processed
in the ir.OCALL case. Those aren't handled yet.

Change-Id: I9a6a4c572e3c54a8e8efad98365184dbb94c4487
Reviewed-on: https://go-review.googlesource.com/c/go/+/325330


Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>