cmd/compile/internal/gc: remove unused state.placeholder field
cmd/compile/internal/gc: remove unused state.placeholder field

gc.state.placeholder was added in 5a6e511c61 but never used.

Change-Id: I5a621507279d5bb1f3991b7a412d9a63039c464e
Reviewed-on: https://go-review.googlesource.com/38755
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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