cmd/compile: separate out sparsemaps that need position

cmd/compile: separate out sparsemaps that need position

Make them a separate type, so the normal sparse maps don't
need the extra storage.

Change-Id: I3a0219487c35ea63723499723b0c742e321d97c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/444819
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>