[dev.typeparams] cmd/compile: swap export order of union term components (cleanup)

Export a term as a pair (tilde, type) rather than (type, tilde)
to match the new Union/Term API.

Change-Id: I221c09c2c746ae19fbae0c970ffb26fa7a8ac736
Reviewed-on: https://go-review.googlesource.com/c/go/+/340251


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