cmd/compile: add ZeroWB case in writebarrier

It looks like it should be there, although I couldn't find a test
case that fails without it. ZeroWB is probably never generated now:
zeroing an initialized heap object is done by making an autotmp on
stack, zeroing it, and copying (typedmemmove) to heap.

Passes "toolstash -cmp" on std.

Change-Id: I702a59759e33fb8cc2a34a3b3029e7540aca080a
Reviewed-on: https://go-review.googlesource.com/35250


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>