s390: Fix s390_constant_via_vgbm_p() [PR118362]

s390: Fix s390_constant_via_vgbm_p() [PR118362]

Optimization s390_constant_via_vgbm_p() should only apply to constant
vectors which can be expressed by the hardware, i.e., which have a size
of at most 16-bytes, similar as it is done for s390_constant_via_vgm_p()
and s390_constant_via_vrepi_p().

gcc/ChangeLog:

	PR target/118362
	* config/s390/s390.cc (s390_constant_via_vgbm_p): Allow at most
	16-byte vectors.