* config/s390/s390.c (legitimate_reload_constant_p): Accept floating-
point zero operands that fit into a single GPR.
(s390_preferred_reload_class): Ensure we only return general-purpose
register classes.
* config/s390/s390.md ("*mov<mode>_64dfp"): Use lghi to load
floating-point zero operands into GPRs.
("*mov<mode>_64"): Likewise.
("mov<mode>"): Likewise using lhi.