[dev.ssa] cmd/compile: don't alloc new CSE classes

[dev.ssa] cmd/compile: don't alloc new CSE classes

This reduces the time to compile
test/slice3.go on my laptop from ~12s to ~3.8s.
It reduces the max memory use from ~4.8gb to
~450mb.

This is still considerably worse than tip,
at 1s and 300mb respectively, but it's
getting closer.

Hopefully this will fix the build at long last.

Change-Id: Iac26b52023f408438cba3ea1b81dcd82ca402b90
Reviewed-on: https://go-review.googlesource.com/12566
Reviewed-by: Keith Randall <khr@golang.org>