Help compiler exploit hanging-node symmetries by reduced reg loads

[kronbichler]

This is a pure performance optimization: The current vectorized code path of the hanging-node evaluation routines exploits the symmetry in shape functions between the two subface interpolation matrices. However, we leave it up to the compiler to actually exploit this by fewer memory-register moves, which is not done for high degrees as there are too many loads in between the two places where the data is re-used (whereas at least clang-13 does the optimization for p=3, so I did not notice immediately). Hence, make the loop explicit, similar to what we do in

dealii/include/deal.II/matrix_free/tensor_product_kernels.h

Lines 1155 to 1160 in 43550bc

	in0 = in[stride * ind];
	in1 = in[stride * (mm - 1 - ind)];
	res0 += val0 * in0;
	res1 += val1 * in0;
	res0 += val1 * in1;
	res1 += val0 * in1;