If-condition could cause performance issues with CurvedMesh

[efaulhaber]

In #514, the code

if left_element <= 0 # left_element = 0 at boundaries
  return surface_flux_values
end

is introduced in solvers/dg_curved/dg_2d.jl.

This is quite a bit slower than without the condition for LSA, but for some reason a bit faster for compressible Euler.

This benchmark can be used to investigate the difference:

using Trixi
using BenchmarkTools
trixi_include("examples/2d/elixir_euler_source_terms_curved.jl")

u = Trixi.wrap_array(sol.u[begin], semi)

@benchmark Trixi.calc_interface_flux!($u, $mesh, $equations, $solver, $semi.cache)