[dev.ssa] cmd/compile/internal/ssa: handle rewrite of Phis.

[dev.ssa] cmd/compile/internal/ssa: handle rewrite of Phis.

* Phis can have variable number of arguments, but rulegen assumed that
each operation has fixed number of arguments.
* Rewriting Phis is necessary to handle the following case:

func f1_ssa(a bool, x int) int {
        v := 0
        if a {
                v = -1
        } else {
                v = -1
        }
        return x|v
}

Change-Id: Iff6bd411b854f3d1d6d3ce21934bf566757094f2
Reviewed-on: https://go-review.googlesource.com/19412
Reviewed-by: Keith Randall <khr@golang.org>