[llvm] [RegAllocFast] Refactor dominates algorithm for large basic block (PR #72250)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This `InstrPosIndexes` optimized `dominates()` from 
[RegAllocFast: Make self loop live-out heuristic more aggressive](https://github.com/llvm/llvm-project/commit/738c73a454881ca78214816754c1b82941d0cd26) (2010), used to optimize https://reviews.llvm.org/D52010 ("RegAllocFast: Rewrite and improve"). I have a WIP to make RegAllocFast consume SSA MachineIR (absoring PHIElimination and TwoAddressInstruction), which can eventually remove the code here.

https://github.com/llvm/llvm-project/pull/72250