Fixed overshifting an expression by a constant so that we overshift to
zero. A test case was added for this.

In addition the use to vc_bvExtract()  was removed for shifting left by an
expression because we don't want/need bitmasked behaviour anymore.
Added C test case that checks that concrete and symbolic overshift
behaviour for left shift are identical.
Fixed overshift of logical right shift by symbolic so that it overshifts
to zero. Test case is included.
Fixed overshift of logical right shift by constant so that it
overshifts to zero. Test case is included.
Added C test case that checks that concrete and symbolic overshift
behaviour for logical right shift are identical.