[dev.ssa] cmd/compile: schedule values dependent on the control later

[dev.ssa] cmd/compile: schedule values dependent on the control later

To reduce the number of spills, give any non-phi values whose argument
is the control the same priority as the control.

With mask.bash, this reduces regenerated flags from 603 to 240.

Change-Id: I26883d69e80357c56b343428fb528102b3f26e7a
Reviewed-on: https://go-review.googlesource.com/14042
Reviewed-by: Keith Randall <khr@golang.org>