maybe_set_first_label_num can take an rtx_code_label *

The function maybe_set_first_label_num acts on a CODE_LABEL; we can
capture that in the type system.

gcc/ChangeLog:
	* emit-rtl.c (maybe_set_first_label_num): Strengthen param from
	rtx to rtx_code_label *.
	* rtl.h (maybe_set_first_label_num): Likewise.

From-SVN: r235525