cmd/compile: fix export/import of range loop.

As with other recent issues, the Init field of a range loop was not
being handled properly. Generally, it is much better to explicitly
import/export the Init statements, else they are incorrectly added
before the associated node, rather than as the Init value of the node.
This was causing labels to not be correctly added to the range loop that
it is immediately preceding.

Made the ORANGE handling completely similar to the OFOR handling.

Fixes #48462

Change-Id: I999530e84f9357f81deaa3dda50660061f710e7c
Reviewed-on: https://go-review.googlesource.com/c/go/+/350911


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>