cmd/compile: make inlining intrinsics cost one

cmd/compile: make inlining intrinsics cost one

We were assigning a cost of 2 for intrinsics:
One when we recognized an intrinsic,
and one for the OCALLFUNC node.

I believe that the intent was that intrinsics should
cost 1, since they are typically an arithmetic op,
and because they tend to occur in performance-sensitive code.
(Not that any of this is particularly principled right now.)

Stop charging when we recognize an intrinsic;
let the OCALLFUNC node cover the cost.

This has a negligible impact on std+cmd.

Change-Id: Ie6ae5c18da3ae7e40aec425aed95c6999c2831a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/221357
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>