Improve cost computation for single-bit bit insertions.

Improve cost computation for single-bit bit insertions.

Some miscomputation of rtx_costs lead to sub-optimal code for
single-bit bit insertions.  This patch implements TARGET_INSN_COST,
which has a chance to see the whole insn during insn combination;
in partictlar the SET_DEST of (set (zero_extract (...) ...)).

gcc/
	* config/avr/avr.cc (avr_insn_cost): New static function.
	(TARGET_INSN_COST): Define to that function.