cmd/compile/internal/gc: simplify typecheck's Efoo consts

cmd/compile/internal/gc: simplify typecheck's Efoo consts

There's no need for Eiota, Eindir, Eaddr, or Eproc; the values are
threaded through to denote various typechecking contexts, but they
don't actually influence typechecking behavior at all.

Also, while here, switch the Efoo const declarations to use iota.

Passes toolstash -cmp.

Change-Id: I5cea869ccd0755c481cf071978f863474bc9c1ed
Reviewed-on: https://go-review.googlesource.com/22271
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>