cmd/compile: remove unnecessary FuncType cloning
cmd/compile: remove unnecessary FuncType cloning

Since FuncTypes are represented as structs rather than linking the
parameter lists together, we no longer need to worry about duplicating
the parameter lists.

Change-Id: I3767aa3cd1cbeddfb80a6eef6b42290dc2ac14ae
Reviewed-on: https://go-review.googlesource.com/28574
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>

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