rs6000: Delete RS6000_CONSTRAINT_f

rs6000: Delete RS6000_CONSTRAINT_f

Constraint "f" is always exactly the same as constraint "d", so we do
not need RS6000_CONSTRAINT_f.

2022-05-11  Segher Boessenkool  <segher@kernel.crashing.org>

	* config/rs6000/constraints.md (register_constraint "f"): Use
	RS6000_CONSTRAINT_d.
	* config/rs6000/rs6000.cc (rs6000_debug_reg_global): Do not handle
	RS6000_CONSTRAINT_f.
	(rs6000_init_hard_regno_mode_ok): Ditto.  Reorder and simplify a bit.
	* config/rs6000/rs6000.h (r6000_reg_class_enum): Delete
	RS6000_CONSTRAINT_d.