AVR: target/117726 - Better optimize shifts.

This patch splits 2-byte and 3-byte shifts after reload into
a 3-operand byte shift and a residual 2-operand shift.

The "2op" shift insn alternatives are not needed and removed because
all shift insn already have a "r,0,n" alternative that does the job.

	PR target/117726
gcc/
	* config/avr/avr-passes.cc (avr_shift_is_3op, avr_emit_shift):
	Also handle 2-byte and 3-byte shifts.
	(avr_split_shift4, avr_split_shift3, avr_split_shift2): New
	local helper functions.
	(avr_split_shift): Use them.
	* config/avr/avr-passes.def (avr_pass_split_after_peephole2):
	Adjust comments.
	* config/avr/avr.cc (avr_out_ashlpsi3, avr_out_ashrpsi3)
	(avr_out_lshrpsi3): Support offset 15.
	(ashrhi3_out): Support offset 7 as 3-op.
	(ashrsi3_out): Support offset 15.
	(avr_rtx_costs_1): Adjust shift costs.
	* config/avr/avr.md (2op): Remove attribute value and all such insn
	alternatives.
	(ashlhi3, *ashlhi3, *ashlhi3_const): Add 3-op alternatives like C2l.
	(ashrhi3, *ashrhi3, *ashrhi3_const): Add 3-op alternatives like C2a.
	(lshrhi3, *lshrhi3, *lshrhi3_const): Add 3-op alternatives like C2r.
	(*ashlpsi3_split, *ashlpsi3): Add 3-op alternatives C15 and C3l.
	(*ashrpsi3_split, *ashrpsi3): Add 3-op alternatives C15 and C3r.
	(*lshrpsi3_split, *lshrpsi3): Add 3-op alternatives C15 and C3r.
	(ashlsi3, *ashlsi3, *ashlsi3_const): Remove "2op" alternative.
	(ashrsi3, *ashrsi3, *ashrsi3_const): Same.
	(lshrsi3, *lshrsi3, *lshrsi3_const): Same.
	(constr_split_suffix): Code attr morphed from constr_split_shift4.
	* config/avr/constraints.md (C2a, C2r, C2l)
	(C3a, C3r, C3l): New constraints.
	* doc/invoke.texi (AVR Options) <-msplit-bit-shift>: Adjust doc.

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