[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 <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

theif519/go,mwhudson/go,josharian/go,vanloswang/go,Cofyc/go,akutz/go,nawawi/go,4ad/go,polyverse-security/go,CAFxX/go,goodlang/good,binarycrusader/go,suncycheng/go,golang/go,momchil-velikov/go,derekmarcotte/go,cloudflare/go,vanloswang/go,cloudflare/go,cyberroadie/go,vsekhar/elastic-go,4ad/go,josharian/go,vsekhar/elastic-go,nawawi/go,binarycrusader/go,christopher-henderson/Go,theif519/go,golang/go,momchil-velikov/go,Cofyc/go,josharian/go,ImJasonH/go,golang/go,vanloswang/go,polyverse-security/go,miolini/go,theif519/go,linux-on-ibm-z/go,nawawi/go,vanloswang/go,cloudflare/go,golang/go,ImJasonH/go,akutz/go,momchil-velikov/go,suncycheng/go,josharian/go,ganboing/go-esx,goodlang/good,josharian/go,goodlang/good,akutz/go,vsekhar/elastic-go,polyverse-security/go,linux-on-ibm-z/go,4ad/go,codestation/go,polyverse-security/go,cloudflare/go,Samurais/go,ganboing/go-esx,CAFxX/go,mwhudson/go,josharian/go,binarycrusader/go,ganboing/go-esx,polyverse-security/go,ImJasonH/go,codestation/go,binarycrusader/go,derekmarcotte/go,nawawi/go,momchil-velikov/go,akutz/go,linux-on-ibm-z/go,akutz/go,josharian/go,goodlang/good,golang/go,suncycheng/go,codestation/go,Samurais/go,ganboing/go-esx,cyberroadie/go,miolini/go,4ad/go,Cofyc/go,nawawi/go,4ad/go,derekmarcotte/go,cyberroadie/go,derekmarcotte/go,akutz/go,polyverse-security/go,goodlang/good,4ad/go,cloudflare/go,derekmarcotte/go,suncycheng/go,ganboing/go-esx,theif519/go,suncycheng/go,golang/go,ImJasonH/go,goodlang/good,cloudflare/go,theif519/go,binarycrusader/go,nawawi/go,momchil-velikov/go,goodlang/good,mwhudson/go,CAFxX/go,ImJasonH/go,suncycheng/go,akutz/go,suncycheng/go,cyberroadie/go,vanloswang/go,golang/go,miolini/go,ganboing/go-esx,linux-on-ibm-z/go,akutz/go,nawawi/go,codestation/go,vanloswang/go,Samurais/go,Samurais/go,codestation/go,Cofyc/go,ImJasonH/go,ganboing/go-esx,goodlang/good,derekmarcotte/go,theif519/go,cloudflare/go,vanloswang/go,cloudflare/go,Cofyc/go,theif519/go,mwhudson/go,ganboing/go-esx,suncycheng/go,Cofyc/go,miolini/go,mwhudson/go,polyverse-security/go,vsekhar/elastic-go,miolini/go,Samurais/go,linux-on-ibm-z/go,vsekhar/elastic-go,binarycrusader/go,4ad/go,mwhudson/go,derekmarcotte/go,derekmarcotte/go,linux-on-ibm-z/go,binarycrusader/go,nawawi/go,Samurais/go,cyberroadie/go,Cofyc/go,vsekhar/elastic-go,CAFxX/go,momchil-velikov/go,linux-on-ibm-z/go,CAFxX/go,theif519/go,codestation/go,CAFxX/go,miolini/go,CAFxX/go,vsekhar/elastic-go,Cofyc/go,ImJasonH/go,binarycrusader/go,cyberroadie/go,linux-on-ibm-z/go,josharian/go,vanloswang/go,miolini/go,miolini/go,polyverse-security/go,golang/go,vsekhar/elastic-go,codestation/go,mwhudson/go,Samurais/go,Samurais/go,momchil-velikov/go,momchil-velikov/go,ImJasonH/go,cyberroadie/go,CAFxX/go,cyberroadie/go,mwhudson/go,codestation/go,4ad/go