JIT: Replace `BlockSet` with `BitVec`, and remove BB epoch

[amanasifkhalid]

Part of #107749. Prerequisite for #110026.

• Use postorder number-based BitVecs in RBO and block layout.
• Use bbID-based BitVecs in fgIncorporateProfileData. This runs early enough in the JIT frontend such that I would expect bbIDs and bbNums to be 1:1, so I don't expect any TP impact from this change.

Switch descriptor creation still uses bbNums as a key into a BitVec as a workaround for BB epoch invariants -- I'll try switching this over to bbID in a follow-up to evaluate the TP cost of a sparser bitset.

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[amanasifkhalid]

cc @dotnet/jit-contrib, @jakobbotsch PTAL. No diffs. Thanks!

[amanasifkhalid]

Can we delete BlockSet/and the epoch fields as well now?

Yes. I can include that in this PR, if you'd like.

[amanasifkhalid]

/ba-g linux-x64 build timed out