AVR: Add an insn combine pattern for offset computation.

AVR: Add an insn combine pattern for offset computation.

Computing  uint16_t += 2 * uint8_t  can occur when an offset
into a 16-bit array is computed.  Without this pattern is costs
six instructions: A move (1), a zero-extend (1), a shift (2) and
an addition (2).  With this pattern it costs 4.

gcc/
	* config/avr/avr.md (*addhi3_zero_extend.ashift1): New pattern.
	* config/avr/avr.cc (avr_rtx_costs_1) [PLUS]: Compute its cost.