AVR: target/116295 - Fix unrecognizable insn with __flash read.

Some loads from non-generic address-spaces are performed by
libgcc calls, and they don't have a POST_INC form.  Don't consider
such insns when running -mfuse-add.

     PR target/116295
gcc/
	* config/avr/avr.cc (Mem_Insn::Mem_Insn): Don't consider MEMs
	that are avr_mem_memx_p or avr_load_libgcc_p.

gcc/testsuite/
	* gcc.target/avr/torture/pr116295.c: New test.