Correct handling of constant fields.

	Correct handling of constant fields.
	* gen.c (insn_field_cmp): Tweak comment about neither field
	being an insn_field_string with a cond_eq-to-value condition.
	(insns_bit_useless) <case insn_field_string, case
	decode_find_mixed>: Handle cond_eq-to-value fields as
	insn_field_int.
	* gen-idecode.c (print_idecode_validate): Handle
	insn_field_string cond-equal-to-value fields as insn_field_int.
	* gen-icache.c (print_icache_body): Add comment why constant
	string fields are handled.