cmd/compile: cull dead code

cmd/compile: cull dead code

The special case for ODOTPTR to handle zero-width fields is unneeded.
It is an artifact of the old backend, from which time this code dates.
The Node to SSA converter is careful to insert a nil check.
This is tested in test/nilptr2.go, among other places.

Passes toolstash-check.

Change-Id: I6c1d99f7ff5abdae9aa08ee047dc088a3fe8dc3c
Reviewed-on: https://go-review.googlesource.com/c/148828
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>