shorten_branches takes an rtx_insn

shorten_branches takes an rtx_insn

gcc/
	* output.h (shorten_branches): Strengthen param from rtx to
	rtx_insn *.

	* final.c (shorten_branches): Likewise, renaming param back from
	"uncast_first" to "first", and dropping the checked cast from rtx
	to rtx_insn *.

	* genattr.c (gen_attr): Likewise when writing out the prototype of
	shorten_branches.

From-SVN: r214478