MatrixFree: Use constexpr when selecting evaluator factory

[kronbichler]

Since there is a lot of code involved inside the templated path for selecting an evaluator, we should let the compiler completely disable it when we work with fe_degree == -1 (that picks a precompiled version).

[bangerth]

It surprises me that the compiler doesn't eliminate these code paths altogether, given that the variable tested is a compile-time constant.

[masterleinad]

It surprises me that the compiler doesn't eliminate these code paths altogether, given that the variable tested is a compile-time constant.

The compiler is still required to instantiate and can discard unused code paths in a later pass.