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.

gcc/combine.cc

@@ -10635,8 +10635,10 @@ simplify_shift_const_1 (enum rtx_code code, machine_mode result_mode,
 					     outer_op, outer_const);
 	}
 
-      scalar_int_mode shift_unit_mode
-	= as_a <scalar_int_mode> (GET_MODE_INNER (shift_mode));
+      scalar_int_mode shift_unit_mode;
+      if (!is_a <scalar_int_mode> (GET_MODE_INNER (shift_mode),
+				   &shift_unit_mode))
+	return NULL_RTX;
 
       /* Handle cases where the count is greater than the size of the mode
 	 minus 1.  For ASHIFT, use the size minus one as the count (this can

gcc/testsuite/gcc.dg/fixed-point/pr117611.c

@@ -0,0 +1,7 @@
+/* { dg-do compile } */
+/* { dg-options "-Os" } */
+
+_Accum acc1 (_Accum x)
+{
+    return x << 16;
+}