cmd/compile: use local fn variable in compile
cmd/compile: use local fn variable in compile

fn == Curfn in this context. Prefer the local variable.

Passes toolstash -cmp. Updates #15756.

Change-Id: I75b589c682d0c1b524cac2bbf2bba368a6027b06
Reviewed-on: https://go-review.googlesource.com/38151
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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