* constexpr.c (cxx_eval_check_shift_p): New function.
	(cxx_eval_binary_expression): Call it.  Set NON_CONSTANT_P if it
	returns true.

	* g++.dg/cpp0x/constexpr-shift1.C: New test.
	* g++.dg/cpp1y/constexpr-shift1.C: New test.
	* g++.dg/ubsan/pr63956.C: Add dg-errors.

From-SVN: r218279