[Support] Use block numbers for LoopInfo BBMap (#103400)
Replace the DenseMap from blocks to their innermost loop a vector
indexed by block numbers, when possible. Supporting number updates is
not trivial as we don't store a list of basic blocks, so this is not
implemented.
NB: I'm generally not happy with the way loops are stored. As I think
that there's room for improvement, I don't want to touch the
representation at this point.
Pull Request: https://github.com/llvm/llvm-project/pull/103400
