QV4: Don't heap allocate blockNumbers.

QV4: Don't heap allocate blockNumbers.

Analysis of the allocations in BasicBlockSet show a huge amount of time spent in
allocating and freeing BasicBlockSet, which involves heap allocations. Most of
the time, blockNumbers is tiny (at least in my testing) - 0 to 1 in size at
deletion time. By inlining the (small) array, we save a nasty penalty.

This shaves around 400ms off the optimizer phase of my morbid QML testcase.

Change-Id: I46319173b5408a0d7a1b9663fdc516c9e5ca410e
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>