Make some smallest_int_mode_for_size calls cope with failure

Make some smallest_int_mode_for_size calls cope with failure

smallest_int_mode_for_size now returns an optional mode rather
than aborting on failure.  This patch adjusts a couple of callers
so that they fail gracefully when no mode exists.

There should be no behavioural change, since anything that triggers
the new return paths would previously have aborted.  I just think
this is how the code would have been written if the option had been
available earlier.

gcc/
	* dse.cc (find_shift_sequence): Allow smallest_int_mode_for_size
	to failure.
	* optabs.cc (expand_twoval_binop_libfunc): Likewise.