cmd/compile: tidy up switch case expansion
cmd/compile: tidy up switch case expansion

No functional changes.

Change-Id: I0961227e8a7be2d7c611452896843b6955303fa6
Reviewed-on: https://go-review.googlesource.com/26768
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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