More constexpr helpers

This MR converts some helper functions in XprHelper.h from template metaprogramming to constexpr functions. The change functions are

• compute_default_alignment_helper
• compute_matrix_flags
• size_at_compile_time When replacing usages of the latter, I noticed that in many cases the call site could actually be simplified by using the already existing helper size_of_xpr_at_compile_time, so I've updated these.

In order to get these to compile, I also had to mark ignore_unused_variable as a constexpr function.