[DependencyInjection]Micro-optimization

Bug fix: no
Feature addition: not really
Backwards compatibility break: no
Fixes the following tickets: ~
Todo: see below
License of the code: MIT
Documentation PR: ~

This adds a small micro-optimization to the container once it's dumped as PHP.

Even if the speed bump isn't that visible on the default Symfony2 Standard distribution, this doesn't break anything and it should be helpful when having a lot of parameters in the container.

I've did got better results with this applied but it would be helpful if others could confirm this to be true as well.