cmd/compile/internal: named types for Etype and Op in struct Node

cmd/compile/internal: named types for Etype and Op in struct Node

Type Op is enfored now.
Type EType will need further CLs.
Added TODOs where Node.EType is used as a union type.
The TODOs have the format `TODO(marvin): Fix Node.EType union type.`.

Furthermore:
-The flag of Econv function in fmt.go is removed, since unused.
-Some cleaning along the way, e.g. declare vars first when getting initialized.

Passes go build -toolexec 'toolstash -cmp' -a std.

Fixes #11846

Change-Id: I908b955d5a78a195604970983fb9194bd9e9260b
Reviewed-on: https://go-review.googlesource.com/14956
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>