cmd/compile: create "init" function during noding

cmd/compile: create "init" function during noding

This CL arranges for package-scope initialization statements to be
constructed directly into their eventual "init" function, so we can
eliminate the roundabout solution of using InitTodoFunc.

While here, somewhat simplify and generalize the logic for outlining
map initialization statements.

Change-Id: I8aff042e6b266f7024de436424ec6711b8b69129
Reviewed-on: https://go-review.googlesource.com/c/go/+/522318
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>