JIT: Move hot/cold splitting phase to backend

[amanasifkhalid]

Fixes #108611. Running hot/cold splitting at most once ensures the coldness of blocks (as tracked by BBF_COLD) is correct by the time we get to the emitter.

@dotnet/jit-contrib PTAL, thanks!

[amanasifkhalid]

/azp run Antigen

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[amanasifkhalid]

No diffs. Antigen run didn't repro the failure in #108611. @BruceForstall could you PTAL? Thanks!

[amanasifkhalid]

ping @dotnet/jit-contrib

[kunalspathak]

do you think we should have added the test case for this?

[amanasifkhalid]

I don't think that test case in particular was revealing anything subtle. Looking at earlier Antigen runs, I believe #107483 broke hot/cold splitting pretty substantially, such that if we found a place to split before lowering/LSRA, then lowering/LSRA introduced new blocks, and the new blocks forced us to split later in the method, it would be possible for the emitter to assert later due to jumps being too short. I'm surprised runtime-jit-experimental didn't hit this, though running SPMI with fake hot/cold splitting quickly revealed this issue.

As for the fix, I was planning on moving hot/cold splitting later as part of the flowgraph modernization work -- I just didn't realize that this had correctness implications, too.