AVR: target/118878 - Don't ICE on result from paradoxical reg's alloc.

AVR: target/118878 - Don't ICE on result from paradoxical reg's alloc.

After register allocation, paradoxical subregs may become something
like  r20:SI += r22:SI  which doesn't make much sense as assembly code.
Hence avr_out_plus_1() used to ICE on such code.  However, paradoxical
subregs appear to be a common optimization device (instead of proper
mode demotion).

	PR target/118878
gcc/
	* config/avr/avr.cc (avr_out_plus_1): Don't ICE on result of
	paradoxical reg's register allocation.
gcc/testsuite/
	* gcc.target/avr/torture/pr118878.c: New test.