cmd/compile: move early deadcode into unified writer
This CL moves the early deadcode elimination pass into the unified
writer. This allows shrinking the export data, by simplifying
expressions and removing unreachable statements. It also means we
don't need to repeatedly apply deadcode elimination on inlined calls
or instantiated generics.
Change-Id: I19bdb04861e50815fccdab39790f4aaa076121fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/517775 Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>