If there isn't a value dependency between the control value of a
block and some other value, the schedule pass might move the control
value to a spot that is not EOB. Fix by handling the control value
specially like phis.

Change-Id: Iddaf0924d98c5b3d9515c3ced927b0c85722818c
Reviewed-on: https://go-review.googlesource.com/11071Reviewed-by: Keith Randall <khr@golang.org>