asan: score: global-buffer-overflow

I'm flying blind here, not having an s+core s3 insn set reference,
but this seems reasonably obvious from what is done by the assembler.
s3_do16_rpop does some mixing of imm and reg values to place in the
rpop reg field, but I'm not going to try to fix the disassembly
there.

	* score-dis.c (print_insn_score16): Move rpush/rpop imm field
	value adjustment so that it doesn't affect reg field too.