JIT: implement tail merging (#77103)

Add a phase that looks for common tail statements in a block's
predecessors and merges them.

Run it both before and after morph.

Also
* add range enable config and overall `JitEnableTailMerge` config
* add indir flag checking to `GenTree::Compare`
* remove an apparently unnecessary assert from loop recognition.

Closes #8795.
Closes #76872.

src/coreclr/jit/compiler.cpp

@@ -4549,6 +4549,10 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     //
     if (opts.OptimizationEnabled())
     {
+        // Tail merge
+        //
+        DoPhase(this, PHASE_TAIL_MERGE, &Compiler::fgTailMerge);
+
         // Merge common throw blocks
         //
         DoPhase(this, PHASE_MERGE_THROWS, &Compiler::fgTailMergeThrows);
@@ -4650,6 +4654,10 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
         //
         DoPhase(this, PHASE_OPTIMIZE_FLOW, &Compiler::optOptimizeFlow);
 
+        // Second pass of tail merge
+        //
+        DoPhase(this, PHASE_TAIL_MERGE2, &Compiler::fgTailMerge);
+
         // Compute reachability sets and dominators.
         //
         DoPhase(this, PHASE_COMPUTE_REACHABILITY, &Compiler::fgComputeReachability);

src/coreclr/jit/compiler.h

@@ -5224,6 +5224,8 @@ class Compiler
 
     void fgMoveBlocksAfter(BasicBlock* bStart, BasicBlock* bEnd, BasicBlock* insertAfterBlk);
 
+    PhaseStatus fgTailMerge();
+
     enum FG_RELOCATE_TYPE
     {
         FG_RELOCATE_TRY,    // relocate the 'try' region

src/coreclr/jit/compphases.h

@@ -53,8 +53,10 @@ CompPhaseNameMacro(PHASE_COMPUTE_EDGE_WEIGHTS,       "Compute edge weights (1, f
 #if defined(FEATURE_EH_FUNCLETS)
 CompPhaseNameMacro(PHASE_CREATE_FUNCLETS,            "Create EH funclets",             false, -1, false)
 #endif // FEATURE_EH_FUNCLETS
+CompPhaseNameMacro(PHASE_TAIL_MERGE,                 "Tail merge",                     false, -1, false)
 CompPhaseNameMacro(PHASE_MERGE_THROWS,               "Merge throw blocks",             false, -1, false)
 CompPhaseNameMacro(PHASE_INVERT_LOOPS,               "Invert loops",                   false, -1, false)
+CompPhaseNameMacro(PHASE_TAIL_MERGE2,                "Post-morph tail merge",          false, -1, false)
 CompPhaseNameMacro(PHASE_OPTIMIZE_FLOW,              "Optimize control flow",          false, -1, false)
 CompPhaseNameMacro(PHASE_OPTIMIZE_LAYOUT,            "Optimize layout",                false, -1, false)
 CompPhaseNameMacro(PHASE_COMPUTE_REACHABILITY,       "Compute blocks reachability",    false, -1, false)