cmd/compile/internal/gc: cleanup mkinlcall
cmd/compile/internal/gc: cleanup mkinlcall

I had too many failed attempts trying to remove iterFields that I
decided to overhaul this function. Much simpler and easier to
understand now (at least IMO).

Passes toolstash-check -all.

Change-Id: I41d00642a969698df3f4689e41a386346b966638
Reviewed-on: https://go-review.googlesource.com/39856
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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