cmd/compile: make use of types2.InitOrder

cmd/compile: make use of types2.InitOrder

types2 already computes the order that package-level variables need to
be initialized in. Start using it.

Change-Id: Idf2740f963b8146f7c927f57effdbf245f41d355
Reviewed-on: https://go-review.googlesource.com/c/go/+/517617
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>