[dev.ssa] cmd/compile/internal/ssa: ensure Phi ops are scheduled first

[dev.ssa] cmd/compile/internal/ssa: ensure Phi ops are scheduled first

Phi ops should always be scheduled first.  They have the semantics
of all happening simultaneously at the start of the block.  The regalloc
phase assumes all the phis will appear first.

Change-Id: I30291e1fa384a0819205218f1d1ec3aef6d538dd
Reviewed-on: https://go-review.googlesource.com/12154
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>