[dev.ssa] cmd/internal/ssa: implement more compiler passes

[dev.ssa] cmd/internal/ssa: implement more compiler passes

opt:      machine-independent optimization
fuse:     join basic blocks
lower:    convert to machine-dependent opcodes
critical: remove critical edges for register alloc
layout:   order basic blocks
schedule: order values in basic blocks
cgen:     generate assembly output

opt and lower use machine-generated matching rules using
the rule generator in rulegen/

cgen will probably change in the real compiler, as we want to
generate binary directly instead of ascii assembly.

Change-Id: Iedd7ca70f6f55a4cde30e27cfad6a7fa05691b83
Reviewed-on: https://go-review.googlesource.com/7981
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Keith Randall <khr@golang.org>