Currently, zeroing generates an ssa.OpZero, which never has write
barriers, even if the assignment is an OASWB. The hybrid barrier
requires write barriers on zeroing, so change OASWB to generate an
ssa.OpZeroWB when assigning the zero value, which turns into a
typedmemclr.

Updates #17503.

Change-Id: Ib37ac5e39f578447dbd6b36a6a54117d5624784d
Reviewed-on: https://go-review.googlesource.com/31451Reviewed-by: Cherry Zhang <cherryyz@google.com>