Fix altivec regression caused by D115670 in Vec Const Eval

The Vector Constant Evaluator assumes that all the types of its
sub-expressions are going to be Vector APValues, which holds for most
situations.  However, in the 1 examples of Altivec C compilation of
operator ++ (not allowed for other vector types), the result is an
LValue.

Since the operator isn't supported for constant evaluation anyway, this
patch just fails-out of constant eval if we are in a situation where the
operand to the unary operator causes an LValue.