target/110088: Improve operation of l-reg with const after move from d-reg.

target/110088: Improve operation of l-reg with const after move from d-reg.

After reload, there may be sequences like
   lreg = dreg
   lreg = lreg <op> const
with an LD_REGS dreg, non-LD_REGS lreg, and <op> in PLUS, IOR, AND.
If dreg dies after the first insn, it is possible to use
   dreg = dreg <op> const
   lreg = dreg
instead which is more efficient.

gcc/
	PR target/110088
	* config/avr/avr.md: Add an RTL peephole to optimize operations on
	non-LD_REGS after a move from LD_REGS.
	(piaop): New code iterator.