QVector - optimize removeLast (and takeLast + pop_back)

In our benchmarks this makes removeLast more than twice as fast.
(That is on my core I7 laptop with gcc on linux).

It changes the alloc test to be an assert rather than an if.

Change-Id: Id55195b9b7880e54a89be4dd9d6228d94aff23c7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>