Add a bool to opInfo to indicate if an Op never results in any
instructions. This is a conservative approximation: some operations,
like Copy, may or may not generate code depending on their arguments.

I built the list by reading each arch's ssaGenValue function. Hopefully
I got them all.

Change-Id: I130b251b65f18208294e129bb7ddc3f91d57d31d
Reviewed-on: https://go-review.googlesource.com/97957Reviewed-by: Keith Randall <khr@golang.org>