AVR: Create more opportunities for -mfuse-add optimization.

avr_split_tiny_move() was only run for AVR_TINY because it has no PLUS
addressing modes.  Same applies to the X register on ordinary cores, and
also to the Z register when used with [E]LPM.  For example, without this patch

long long addLL (long long *a, long long *b)
{
  return *a + *b;
}

compiles with "-mmcu=atmgea128 -Os -dp" to:

    ...
    movw r26,r24     ;  80  [c=4 l=1]  *movhi/0
    movw r30,r22     ;  81  [c=4 l=1]  *movhi/0
    ld r18,X         ;  82  [c=4 l=1]  movqi_insn/3
    adiw r26,1   ;  83  [c=4 l=3]  movqi_insn/3
    ld r19,X
    sbiw r26,1
    adiw r26,2   ;  84  [c=4 l=3]  movqi_insn/3
    ld r20,X
    sbiw r26,2
    adiw r26,3   ;  85  [c=4 l=3]  movqi_insn/3
    ld r21,X
    sbiw r26,3
    adiw r26,4   ;  86  [c=4 l=3]  movqi_insn/3
    ld r22,X
    sbiw r26,4
    adiw r26,5   ;  87  [c=4 l=3]  movqi_insn/3
    ld r23,X
    sbiw r26,5
    adiw r26,6   ;  88  [c=4 l=3]  movqi_insn/3
    ld r24,X
    sbiw r26,6
    adiw r26,7   ;  89  [c=4 l=2]  movqi_insn/3
    ld r25,X
    ld r10,Z         ;  90  [c=4 l=1]  movqi_insn/3
    ...

whereas with this patch it becomes:

    ...
    movw r26,r24     ;  80  [c=4 l=1]  *movhi/0
    movw r30,r22     ;  81  [c=4 l=1]  *movhi/0
    ld r18,X+        ;  140 [c=4 l=1]  movqi_insn/3
    ld r19,X+        ;  142 [c=4 l=1]  movqi_insn/3
    ld r20,X+        ;  144 [c=4 l=1]  movqi_insn/3
    ld r21,X+        ;  146 [c=4 l=1]  movqi_insn/3
    ld r22,X+        ;  148 [c=4 l=1]  movqi_insn/3
    ld r23,X+        ;  150 [c=4 l=1]  movqi_insn/3
    ld r24,X+        ;  152 [c=4 l=1]  movqi_insn/3
    ld r25,X         ;  109 [c=4 l=1]  movqi_insn/3
    ld r10,Z         ;  111 [c=4 l=1]  movqi_insn/3
    ...

gcc/
	* config/avr/avr.md: Also split with avr_split_tiny_move()
	for non-AVR_TINY.
	* config/avr/avr.cc (avr_split_tiny_move): Don't change memory
	references with base regs that can do PLUS addressing.
	(avr_out_lpm_no_lpmx) [POST_INC]: Don't output final ADIW when the
	address register is unused after.
gcc/testsuite/
	* gcc.target/avr/torture/fuse-add.c: New test.