cmd/compile: fix write barrier control value on wasm

This commit fixes a regression with wasm caused by a367f44c.
It adds optimizations to the lowering rules of wasm to ensure
that the lowered version of the code generated for write barriers
is simple enough so it can be processed by Liveness.markUnsafePoints.

Change-Id: Ic98f0dd3791fe1df23dcb34d2457fbde7ffce441
Reviewed-on: https://go-review.googlesource.com/114375


Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>