cmd/compile: remove some write barriers for stack writes

cmd/compile: remove some write barriers for stack writes

This, along with CL 30140, removes ~50% of stack write barriers
mentioned in issue #17330. The remaining are most due to Phi and
FwdRef, which is not resolved when building SSA. We might be
able to do it at a later stage where Phi and Copy propagations
are done, but matching an if-(store-store-call)+ sequence seems
not very pleasant.

Updates #17330.

Change-Id: Iaa36c7b1f4c4fc3dc10a27018a3b0e261094cb21
Reviewed-on: https://go-review.googlesource.com/30290
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>