cmd/compile/internal/syntax: match argument and parameter parsing (cleanup)
cmd/compile/internal/syntax: match argument and parameter parsing (cleanup)

No semantic change. Move functionality not related to argument
out of the argument parsing function, and thus match parameter
parsing. Also, use a better function name.

Change-Id: Ic550875251d64e6fe1ebf91c11d33a9e4aec9fdd
Reviewed-on: https://go-review.googlesource.com/70491
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

josharian/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,josharian/go,josharian/go,codestation/go,nawawi/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,akutz/go,nawawi/go,josharian/go,golang/go,cloudflare/go,Cofyc/go,codestation/go,cloudflare/go,cloudflare/go,CAFxX/go,akutz/go,cloudflare/go,codestation/go,josharian/go,akutz/go,Cofyc/go,codestation/go,nawawi/go,nawawi/go,akutz/go,Cofyc/go,golang/go,codestation/go,josharian/go,christopher-henderson/Go,Cofyc/go,josharian/go,nawawi/go,cloudflare/go,codestation/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,golang/go,golang/go,nawawi/go,Cofyc/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,akutz/go,Cofyc/go,josharian/go,codestation/go,Cofyc/go,Cofyc/go,akutz/go,codestation/go