s390: Implement vec_extract via vec_select.

vec_select can handle dynamic/runtime masks nowadays.  Therefore we can
get rid of the UNSPEC_VEC_EXTRACT that was preventing further
optimizations like combining instructions with vec_extract patterns.

gcc/ChangeLog:

	* config/s390/s390.md: Remove UNSPEC_VEC_EXTRACT.
	* config/s390/vector.md: Rewrite patterns to use vec_select.
	* config/s390/vx-builtins.md (vec_scatter_element<V_HW_2:mode>_SI):
	Likewise.