cmd/compile: make assignop/convertop reason a return param

On a negative answer, the assignop and convertop functions write the
reason why to a string pointer passed as an argument, likely a C-ism
leftover since the compiler's machine assisted translation to Go.

This change makes why a return parameter.

It also fixes a few places where the assignop/convertop result was
compared to 0. While OXXX's value may be zero now, using the named
constant is more robust.

Change-Id: Id9147ed4c1b97d658d30a2f778f876b7867006b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/261857


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Alberto Donizetti <alb.donizetti@gmail.com>