[dev.ssa] cmd/compile: fix @ rewrite rules

[dev.ssa] cmd/compile: fix @ rewrite rules

The @ directive used to read the target block after some value
structure had already changed.  I don't think it was ever really
a bug, but it's confusing.

It might fail like this:

(Foo x y) -> @v.Args[0].Block (Bar y (Baz ...))

v.Op = Bar
v.Args[0] = y
v.Args[1] = v.Args[0].Block.NewValue(Baz, ...)

That new value is allocated in the block of y, not the
block of x.

Anyway, read the destination block first so this
potential bug can't happen.

Change-Id: Ie41d2fc349b35cefaa319fa9327808bcb781b4e2
Reviewed-on: https://go-review.googlesource.com/19900
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>