AVR: Tweak >= and < compares with consts that are 0 mod 256.

The >= and < comparisons may skip comparing the lower bytes when
the according bytes of the constant are all zeros.  For example,

    uint16 >= 0x1200

is true iff

    hi8 (uint16) >= hi8 (0x1200)

and similar for uint16 < 0x1200.  Some comparisons against constants
that are an integral power of 256 where already handled in the split
preparation.  That code has been outsourced to new avr_maybe_cmp_lsr()
which may change the operands such that the resulting insns become
a comparison of the high bytes against 0 plus a EQ / NE branch.
For example,

    uint32 >= 0x10000

can be rewritten as

    (uint32 >> 16) != 0.

The according asm output is performed by new avr_out_cmp_lsr().

gcc/
	* config/avr/avr-protos.h (avr_out_cmp_lsr, avr_maybe_cmp_lsr): New.
	* config/avr/avr.cc (avr_maybe_cmp_lsr, avr_out_cmp_lsr): New functions.
	(avr_out_compare) [GEU, LTU]: Start output at byte CTZ(xval) / 8.
	(avr_adjust_insn_length) [ADJUST_LEN_CMP_LSR]: Handle case.
	* config/avr/avr.md (adjust_len) <cmp_lsr>: New attr value.
	(*cmp<mode>_lsr): New define_insn_and_split.
	(cbranch<mode>4_insn): When splitting, run avr_maybe_cmp_lsr()
	which may map the operands to *cmp<mode>_lsr.
gcc/testsuite/
	* gcc.target/avr/torture/cmp-lsr-i32.c: New test.
	* gcc.target/avr/torture/cmp-lsr-u16.c: New test.
	* gcc.target/avr/torture/cmp-lsr-u24.c: New test.
	* gcc.target/avr/torture/cmp-lsr-u32.c: New test.
	* gcc.target/avr/torture/cmp-lsr-u64.c: New test.

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