Fix altivec regression caused by D115670 in Vec Const Eval

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.