c++: ICE when shortening right shift [PR94955]

c++: ICE when shortening right shift [PR94955]

Since r10-6527 fold_for_warn calls maybe_constant_value, which means it
can fold more than it previously could.  In this testcase it means that
cp_build_binary_op/RSHIFT_EXPR set short_shift because now we were able
to fold op1 to an INTEGER_CST.  But then when actually performing the
shortening we crashed because cp_fold_rvalue wasn't able to fold as much
as f_f_w and so tree_int_cst_sgn crashed on a NOP_EXPR.  Therefore the
calls should probably match.

	PR c++/94955
	* typeck.c (cp_build_binary_op): Use fold_for_warn instead of
	cp_fold_rvalue.

	* g++.dg/cpp0x/constexpr-shift2.C: New test.