[IR] Add per-function numbers to basic blocks (#101052)

Every basic block that is linked into a function now has a unique
number, which can be queried using getNumber(). Numbers are densely
allocated, but not re-assigned on block removal for stability. Block
numbers are intended to be fairly stable and only be updated when
removing a several basic blocks to make sure the numbering doesn't
become too sparse.

To reduce holes in the numbering, renumberBlocks() can be called to
re-assign numbers in block order.

Additionally, getMaxBlockNumber() returns a value larger than the
largest block number, intended to pre-allocate/resize vectors.

Furthermore, this introduces the concept of a "block number epoch" --
an integer that changes after every renumbering. This is useful for
identifying use of block numbers after renumbering: on initialization,
the current epoch is stored, and on all subsequent accesses, equality
with the current epoch can be asserted.

I added a validate method to catch cases where something goes wrong,
even if I can't really imagine how invalid numbers can occur. But I
think it's better to be safe and rule out this potential source of bugs
when more things depend on the numbering.

Previous discussion in:
https://discourse.llvm.org/t/rfc-add-auxiliary-field-for-per-pass-custom-data-to-basicblock/80229

0 files changed, 0 insertions, 0 deletions