combine: Use single_set for is_just_move

combine: Use single_set for is_just_move

Since we now only call is_just_move on the original instructions, we
always have an rtx_insn* (not just a pattern), so we can use single_set
on it.  This makes no detectable difference at all on all thirty Linux
targets I test, but it does help cris, and it is simpler, cleaner code
anyway.

2020-07-16  Hans-Peter Nilsson  <hp@axis.com>
	    Segher Boessenkool  <segher@kernel.crashing.org>

	PR target/93372
	* combine.c (is_just_move): Take an rtx_insn* as argument.  Use
	single_set on it.