Correctly detect shifts out of range

Correctly detect shifts out of range

get_shift_range was incorrectly communicating that it couldn't calculate
a range when the shift values was always out fo range.  Fix this and
alwasy return [0, 0] when the shift value is always out of range.

	PR tree-optimization/108306
	gcc/
	* range-op.cc (operator_lshift::fold_range): Return [0, 0] not
	varying for shifts that are always out of void range.
	(operator_rshift::fold_range): Return [0, 0] not
	varying for shifts that are always out of void range.

	gcc/testsuite/
	* gcc.dg/pr108306.c: New.