AVR: Tidy up subtract-and-zero_extend insns.

AVR: Tidy up subtract-and-zero_extend insns.

There are these insns that subtract and zero-extend where
the subtrahend is zero-extended to the mode of the minuend.
This patch uses one insn (and split) with mode iterators
instead of spelling out each variant individually.
This has the additional benefit that u32 - u24 is also supported,
which previously wasn't.

gcc/
	* config/avr/avr-protos.h (avr_out_minus): New prototype.
	* config/avr/avr.cc (avr_out_minus): New function.
	* config/avr/avr.md (*sub<HISI:mode>3.zero_extend.<QIPSI:mode>)
	(*sub<HISI:mode>3.zero_extend.<QIPSI:mode>_split): New insns.
	(*subpsi3_zero_extend.qi_split): Remove isns_and_split.
	(*subpsi3_zero_extend.hi_split): Remove insn_and_split.
	(*subhi3_zero_extend1_split): Remove insn_and_split.
	(*subsi3_zero_extend_split): Remove insn_and_split.
	(*subsi3_zero_extend.hi_split): Remove insn_and_split.
	(*subpsi3_zero_extend.qi): Remove insn.
	(*subpsi3_zero_extend.hi): Remove insn.
	(*subhi3_zero_extend1): Remove insn.
	(*subsi3_zero_extend): Remove insn.
	(*subsi3_zero_extend.hi): Remove insn.
gcc/testsuite/
	* gcc.target/avr/torture/sub-zerox.c: New test.