Move common code from various ISel backends into a common base-class.

Move common code from various ISel backends into a common base-class.

Note: all warnings are due to backends that do not fully implement the
required virtual methods. Or, rephrasing: these backends need work to
get them up-to-speed/quality for all methods that do have warnings.

Change-Id: Ib713f3d76832af42ebe893ad2896eec4bdd4bccb
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>