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/35250Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/writebarrier.go

@@ -35,7 +35,7 @@ func writebarrier(f *Func) {
 	valueLoop:
 		for i, v := range b.Values {
 			switch v.Op {
-			case OpStoreWB, OpMoveWB, OpMoveWBVolatile:
+			case OpStoreWB, OpMoveWB, OpMoveWBVolatile, OpZeroWB:
 				if IsStackAddr(v.Args[0]) {
 					switch v.Op {
 					case OpStoreWB: