Support for cold blocks

Start by locating blockparams_out and the block-ordering and register-allocation entry points that determine bundle merging and split points. Trace how cold blocks could be marked and ordered after normal blocks, then evaluate the impact on RPO-dependent heuristics; done means spills and moves prefer cold blocks without breaking allocation.

It would be useful to be able to mark some blocks as "cold" which means that they are rarely taken cold paths. The register allocator should prefer placing spills and moves in cold blocks if possible.

It turns out that very little needs to be done if we take advantage of the block ordering by requiring all cold blocks to be after normal blocks in terms of block index and instruction indices. This has the following consequences:

• Due to the sorting of blockparams_out, bundle merging will attempt to merge all branch parameters coming from normal blocks before attempting to merge ones coming from cold blocks.
• If a requirement conflict exists within a bundle, the bundle is split at the first conflict point. Since ranges are iterated in order, this will place the split in a cold block if there are no conflicts within normal blocks.
• If register allocation requires splitting a bundle, the first conflict point is used as the split point. Same as above.

My only concern is that the block order will no longer be in RPO which is the ordering recommended by the documentation. While regalloc2 will still function properly, I am less sure of the impact it may have on the heuristics.

Note that there are no requirements related to the ordering of blocks, and there is no requirement that the control flow be reducible. Some heuristics used by the allocator will perform better if the code is reducible and ordered in reverse postorder (RPO), however: in particular, (1) this interacts better with the contiguous-range-of-instruction-indices live range representation that we use, and (2) the "approximate loop depth" metric will actually be exact if both these conditions are met.