Avoid using values that do not dominate the block the Zero op is in.
Should fix the SSA check builder.

The additional OffPtr ops inserted by these rules should always be
optimized away when the Load is replaced with a const zero.

Fixes #25288.

Change-Id: I4163b58e60364f77c8a206ba084073a58ca6320a
Reviewed-on: https://go-review.googlesource.com/112136
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>