rtl-optimization/117611 - ICE in simplify_shift_const_1

The following checks we have a scalar int shift mode before
enforcing it.  As AVR shows the mode can be a signed _Accum mode
as well.

	PR rtl-optimization/117611
	* combine.cc (simplify_shift_const_1): Bail if not
	scalar int mode.

	* gcc.dg/fixed-point/pr117611.c: New testcase.