cmd/compile/internal/gc: steps towards work-queue

cmd/compile/internal/gc: steps towards work-queue

This CL reorganizes the top-level functions for handling package-level
declarations, runtime type descriptors, and SSA compilation to work in
a loop. This generalizes the loop that previously existed in dumpdata.

Change-Id: I0e51e60f6ef9e7f96a4a3ccd5801f7baf83eba9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/520611
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>