AVR: rtl-optimization/121340 - New mini-pass to undo superfluous moves from insn combine.

Insn combine may come up with superfluous reg-reg moves, where the combine
people say that these are no problem since reg-alloc is supposed to optimize
them.  The issue is that the lower-subreg pass sitting between combine and
reg-alloc may split such moves, coming up with a zoo of subregs which are
only handled poorly by the register allocator.

This patch adds a new avr mini-pass that handles such cases.

As an example, take

int f_ffssi (long x)
{
    return __builtin_ffsl (x);
}

where the two functions have the same interface, i.e. there are no extra
moves required for the argument or for the return value. However,

$ avr-gcc -S -Os -dp -mno-fuse-move ...

f_ffssi:
	mov r20,r22	 ;  29	[c=4 l=1]  movqi_insn/0
	mov r21,r23	 ;  30	[c=4 l=1]  movqi_insn/0
	mov r22,r24	 ;  31	[c=4 l=1]  movqi_insn/0
	mov r23,r25	 ;  32	[c=4 l=1]  movqi_insn/0
	mov r25,r23	 ;  33	[c=4 l=4]  *movsi/0
	mov r24,r22
	mov r23,r21
	mov r22,r20
	rcall __ffssi2	 ;  34	[c=16 l=1]  *ffssihi2.libgcc
	ret		 ;  37	[c=0 l=1]  return

where all the moves add up to a no-op.  The -mno-fuse-move option
stops any attempts by the avr backend to clean up that mess.

	PR rtl-optimization/121340
gcc/
	* config/avr/avr.opt (-mfuse-move2): New option.
	* config/avr/avr-passes.def (avr_pass_2moves): Insert after combine.
	* config/avr/avr-passes.cc (make_avr_pass_2moves): New function.
	(pass_data avr_pass_data_2moves): New static variable.
	(avr_pass_2moves): New rtl_opt_pass.
	* config/avr/avr-protos.h (make_avr_pass_2moves): New proto.
	* common/config/avr/avr-common.cc
	(default_options avr_option_optimization_table) <-mfuse-move2>:
	Set for -O1 and higher.
	* doc/invoke.texi (AVR Options) <-mfuse-move2>: Document.

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