Flagalloc was recalculating flags is some situations
when it didn't need to.  Fixed by using the same name
for the original flag calculation instruction throughout.

Change-Id: Ic0bf58f728a8d87748434dd25a67b0708755e1f8
Reviewed-on: https://go-review.googlesource.com/19237
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>