cmd/compile: split genCaseClauses by switch type
cmd/compile: split genCaseClauses by switch type

The implementations are going to start diverging more.
Instead of more if clauses and empty parameters,
specialize.

Change-Id: I44584450592e8c9f72a10d8ada859c07e9d9aa19
Reviewed-on: https://go-review.googlesource.com/26764
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

golang/go,derekmarcotte/go,golang/go,cloudflare/go,CAFxX/go,goodlang/good,Cofyc/go,polyverse-security/go,suncycheng/go,CAFxX/go,momchil-velikov/go,Cofyc/go,akutz/go,CAFxX/go,codestation/go,nawawi/go,derekmarcotte/go,polyverse-security/go,nawawi/go,ImJasonH/go,Cofyc/go,golang/go,linux-on-ibm-z/go,vsekhar/elastic-go,vsekhar/elastic-go,codestation/go,polyverse-security/go,vsekhar/elastic-go,nawawi/go,josharian/go,momchil-velikov/go,codestation/go,cloudflare/go,derekmarcotte/go,ImJasonH/go,cloudflare/go,momchil-velikov/go,nawawi/go,golang/go,suncycheng/go,josharian/go,momchil-velikov/go,momchil-velikov/go,polyverse-security/go,goodlang/good,linux-on-ibm-z/go,nawawi/go,christopher-henderson/Go,akutz/go,ImJasonH/go,linux-on-ibm-z/go,goodlang/good,codestation/go,cloudflare/go,goodlang/good,Cofyc/go,cloudflare/go,vsekhar/elastic-go,cloudflare/go,goodlang/good,CAFxX/go,goodlang/good,derekmarcotte/go,cloudflare/go,josharian/go,derekmarcotte/go,polyverse-security/go,linux-on-ibm-z/go,linux-on-ibm-z/go,nawawi/go,ImJasonH/go,ImJasonH/go,polyverse-security/go,codestation/go,vsekhar/elastic-go,linux-on-ibm-z/go,josharian/go,vsekhar/elastic-go,josharian/go,golang/go,derekmarcotte/go,momchil-velikov/go,akutz/go,suncycheng/go,polyverse-security/go,derekmarcotte/go,suncycheng/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,Cofyc/go,goodlang/good,suncycheng/go,CAFxX/go,suncycheng/go,josharian/go,goodlang/good,momchil-velikov/go,akutz/go,Cofyc/go,Cofyc/go,cloudflare/go,suncycheng/go,golang/go,akutz/go,Cofyc/go,golang/go,CAFxX/go,akutz/go,vsekhar/elastic-go,vsekhar/elastic-go,suncycheng/go,golang/go,codestation/go,nawawi/go,CAFxX/go,codestation/go,momchil-velikov/go,linux-on-ibm-z/go,akutz/go,ImJasonH/go,ImJasonH/go,josharian/go,codestation/go,linux-on-ibm-z/go,CAFxX/go,josharian/go,nawawi/go,akutz/go