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

[Matt Arsenault via llvm-commits]

================
@@ -62,6 +62,71 @@ static RegisterRegAlloc fastRegAlloc("fast", "fast register allocator",
 
 namespace {
 
+/// Assign ascending index for instructions in machine basic block. The index
+/// can be used to determine dominance between instructions in same MBB.
+class InstrPosIndexes {
+public:
+  void init(const MachineBasicBlock &MBB) {
+    CurMBB = &MBB;
+    Instr2PosIndex.clear();
+    uint64_t LastIndex = 0;
+    for (const MachineInstr &MI : MBB) {
+      LastIndex += InstrDist;
+      Instr2PosIndex[&MI] = LastIndex;
+    }
+  }
+
+  /// Set \p Index to index of \p MI. If \p MI is new inserted, it try to assign
+  /// index without affecting existing instruction's index. Return true if all
+  /// instructions index has been reassigned.
+  bool getIndex(const MachineInstr &MI, uint64_t &Index) {
+    assert(MI.getParent() == CurMBB && "MI is not in CurMBB");
+    if (Instr2PosIndex.count(&MI)) {
+      Index = Instr2PosIndex[&MI];
+      return false;
+    }
+
+    unsigned Distance = 1;
+    MachineBasicBlock::const_iterator Start = MI.getIterator(),
+                                      End = std::next(Start);
+    while (Start != CurMBB->begin() &&
+           !Instr2PosIndex.count(&*std::prev(Start))) {
+      --Start;
----------------
arsenm wrote:

Debug instructions aren't quite assigned normally.  They're tracked and fixed up after the block is processed if the register wasn't already assigned 

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