cmd/compile: count init functions from 0, not 1
    
    While we're here, do minor style cleanup.
    Also, since we know exactly how many init
    functions there are, use that knowledge.
    
    This is cleanup prior to a more substantive CL.
    
    Change-Id: I2bba60b3c051c852590f798f45e8268f8bc54ca8
    Reviewed-on: https://go-review.googlesource.com/41499
    Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>