[dev.ssa] cmd/compile: don't flush a value derived from the current value

[dev.ssa] cmd/compile: don't flush a value derived from the current value

If flushing a value from a register that might be used by the current
old-schedule value, save it to the home location.

This resolves the error that was changed from panic to unimplemented in
CL 12655.

Change-Id: If864be34abcd6e11d6117a061376e048a3e29b3a
Reviewed-on: https://go-review.googlesource.com/12682
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>