x86: Add a "CondInst" format for conditionally decoded instructions.

A condition can be specified which will tell the decoder whether to return
the instruction being requested, or, if the condition fails, UD2.

Change-Id: I0f1c075deb10754ce1dd88be1726a196294e41fd
Reviewed-on: https://gem5-review.googlesource.com/4580
Reviewed-by: Michael LeBeane <Michael.Lebeane@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>