cmd/compile: reduce inline cost of OCONVOP

OCONVOP doesn't have effect in the compiled code so, it can be safely
excluded from inline cost calculation.

Also make sequence ODEREF OCONVNOP* OADDR cost 1. This is rather common
conversion, such as *(*uint32)(unsafe.Pointer(&x)).

Fixes #42788

Change-Id: I5001f7e89d985c198b6405694cdd5b819cf3f47a
Reviewed-on: https://go-review.googlesource.com/c/go/+/281232


Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Elias Naur <mail@eliasnaur.com>