[scudo] Ensure all blocks are put in the correct group

While populating new blocks, we didn't always put them into their own
groups because that needs additional sort for an almost-sorted new
blocks array. However, ensuring all blocks are placed in the right group
enables the fast identifying of unused pages in a group by simply
accouting the number of free blocks are there. Therefore, this commit is
used to set up the invariant for future optimizations.

Differential Revision: https://reviews.llvm.org/D141957