rtl-ssa: Fix a silly typo

s/ref/reg/ on a previously unused function name.

gcc/
	* rtl-ssa/functions.h (function_info::ref_defs): Rename to...
	(function_info::reg_defs): ...this.
	* rtl-ssa/member-fns.inl (function_info::ref_defs): Rename to...
	(function_info::reg_defs): ...this.