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 <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

akutz/go,golang/go,CAFxX/go,akutz/go,josharian/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,nawawi/go,josharian/go,nawawi/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,golang/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,nawawi/go,josharian/go,nawawi/go,akutz/go,akutz/go,golang/go,josharian/go,CAFxX/go,nawawi/go,golang/go,golang/go,golang/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go