[dev.ssa] cmd/compile: make cse faster

[dev.ssa] cmd/compile: make cse faster

It is one of the slowest compiler phases right now, and we
run two of them.

Instead of using a map to make the initial partition, use a sort.
It is much less memory intensive.

Do a few optimizations to avoid work for size-1 equivalence classes.

Implement -N.

Change-Id: I1d2d85d3771abc918db4dd7cc30b0b2d854b15e1
Reviewed-on: https://go-review.googlesource.com/19024
Reviewed-by: David Chase <drchase@google.com>