Add an early TailMerge pass (#2721)

* Add an early `TailMerge` pass

Some translations can have a lot of guest calls and since for each guest
call there is a call guard which may return. This can produce a lot of
epilogue code for returns. This pass merges the epilogue into a single
block.

```
Using filter 'hcq'.
Using metric 'code size'.

Total diff: -1648111 (-7.19 %) (bytes):
  Base: 22913847
  Diff: 21265736

Improved: 4567, regressed: 14, unchanged: 144
```

* Set PTC version

* Address feedback

* Handle `void` returning functions

* Actually handle `void` returning functions

* Fix `RegisterToLocal` logging

ARMeilleure/CodeGen/Optimizations/TailMerge.cs

@@ -0,0 +1,83 @@
+using ARMeilleure.IntermediateRepresentation;
+using ARMeilleure.Translation;
+using static ARMeilleure.IntermediateRepresentation.Operation.Factory;
+
+namespace ARMeilleure.CodeGen.Optimizations
+{
+    static class TailMerge
+    {
+        public static void RunPass(in CompilerContext cctx)
+        {
+            ControlFlowGraph cfg = cctx.Cfg;
+
+            BasicBlock mergedReturn = new(cfg.Blocks.Count);
+
+            Operand returnValue;
+            Operation returnOp;
+
+            if (cctx.FuncReturnType == OperandType.None)
+            {
+                returnValue = default;
+                returnOp = Operation(Instruction.Return, default);
+            }
+            else
+            {
+                returnValue = cfg.AllocateLocal(cctx.FuncReturnType);
+                returnOp = Operation(Instruction.Return, default, returnValue);
+            }
+
+            mergedReturn.Frequency = BasicBlockFrequency.Cold;
+            mergedReturn.Operations.AddLast(returnOp);
+
+            for (BasicBlock block = cfg.Blocks.First; block != null; block = block.ListNext)
+            {
+                Operation op = block.Operations.Last;
+
+                if (op != default && op.Instruction == Instruction.Return)
+                {
+                    block.Operations.Remove(op);
+
+                    if (cctx.FuncReturnType == OperandType.None)
+                    {
+                        PrepareMerge(block, mergedReturn);
+                    }
+                    else
+                    {
+                        Operation copyOp = Operation(Instruction.Copy, returnValue, op.GetSource(0));
+
+                        PrepareMerge(block, mergedReturn).Append(copyOp);
+                    }
+                }
+            }
+
+            cfg.Blocks.AddLast(mergedReturn);
+            cfg.Update();
+        }
+
+        private static BasicBlock PrepareMerge(BasicBlock from, BasicBlock to)
+        {
+            BasicBlock fromPred = from.Predecessors.Count == 1 ? from.Predecessors[0] : null;
+
+            // If the block is empty, we can try to append to the predecessor and avoid unnecessary jumps.
+            if (from.Operations.Count == 0 && fromPred != null)
+            {
+                for (int i = 0; i < fromPred.SuccessorsCount; i++)
+                {
+                    if (fromPred.GetSuccessor(i) == from)
+                    {
+                        fromPred.SetSuccessor(i, to);
+                    }
+                }
+
+                // NOTE: `from` becomes unreachable and the call to `cfg.Update()` will remove it.
+                return fromPred;
+            }
+            else
+            {
+                from.AddSuccessor(to);
+
+                return from;
+            }
+        }
+    }
+}