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 <mdempsky@google.com>