[RegAllocFast] Document the allocation algorithm. NFC (#219835) Add a file-level overview: block-local allocation, a stack slot for every value crossing a block boundary, no liveness analysis or coalescing, and a backward walk where uses acquire registers and defs release them. Correct two stale claims: RegUnitStates is indexed by register unit, not by physical register, and allocateInstruction() runs eight phases rather than the two its comment described. Drop a redundant local in reloadAtBegin(), and assert the def operand that isTiedToNotUndef() documents. Aided by Opus 5
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