JIT: remove phis right after optimizing (#110821)

They serve no useful purpose once SSA is gone, and they potentially confuse
other parts of the JIT.

Was also going to make the PHIs zero cost, but this apparently was already
done (not sure when/how).

Closes #88841

src/coreclr/jit/compiler.cpp

@@ -5006,6 +5006,9 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
             // Iterate if requested, resetting annotations first.
             if (opts.optRepeatIteration == opts.optRepeatCount)
             {
+                // If we're done optimizing, just remove the PHIs
+                //
+                fgResetForSsa(/* deepClean */ false);
                 break;
             }
 
@@ -5849,7 +5852,7 @@ void Compiler::ResetOptAnnotations()
 {
     assert(opts.optRepeat);
     assert(JitConfig.JitOptRepeatCount() > 0);
-    fgResetForSsa();
+    fgResetForSsa(/* deepClean */ true);
     vnStore                    = nullptr;
     m_blockToEHPreds           = nullptr;
     m_dominancePreds           = nullptr;

src/coreclr/jit/compiler.h

@@ -5725,7 +5725,7 @@ class Compiler
     PhaseStatus fgSsaBuild();
 
     // Reset any data structures to the state expected by "fgSsaBuild", so it can be run again.
-    void fgResetForSsa();
+    void fgResetForSsa(bool deepClean);
 
     unsigned fgSsaPassesCompleted = 0; // Number of times fgSsaBuild has been run.
     bool     fgSsaValid = false;           // True if SSA info is valid and can be cross-checked versus IR

src/coreclr/jit/ssabuilder.cpp

@@ -15,7 +15,7 @@ PhaseStatus Compiler::fgSsaBuild()
     // If this is not the first invocation, reset data structures for SSA.
     if (fgSsaPassesCompleted > 0)
     {
-        fgResetForSsa();
+        fgResetForSsa(/* deepClean */ true);
     }
 
     SsaBuilder builder(this);
@@ -29,21 +29,36 @@ PhaseStatus Compiler::fgSsaBuild()
     return PhaseStatus::MODIFIED_EVERYTHING;
 }
 
-void Compiler::fgResetForSsa()
+//------------------------------------------------------------------------
+// fgResetForSsa: remove SSA artifacts
+//
+// Arguments:
+//   deepClean - if true, remove all SSA artifacts
+//               if false, just remove PHIs
+//
+// Notes:
+//   deepCleaning is needed in order to rebuild SSA.
+//
+void Compiler::fgResetForSsa(bool deepClean)
 {
-    for (unsigned i = 0; i < lvaCount; ++i)
-    {
-        lvaTable[i].lvPerSsaData.Reset();
-    }
-    lvMemoryPerSsaData.Reset();
-    for (MemoryKind memoryKind : allMemoryKinds())
-    {
-        m_memorySsaMap[memoryKind] = nullptr;
-    }
+    JITDUMP("Removing %s\n", deepClean ? "all SSA artifacts" : "PHI functions");
 
-    if (m_outlinedCompositeSsaNums != nullptr)
+    if (deepClean)
     {
-        m_outlinedCompositeSsaNums->Reset();
+        for (unsigned i = 0; i < lvaCount; ++i)
+        {
+            lvaTable[i].lvPerSsaData.Reset();
+        }
+        lvMemoryPerSsaData.Reset();
+        for (MemoryKind memoryKind : allMemoryKinds())
+        {
+            m_memorySsaMap[memoryKind] = nullptr;
+        }
+
+        if (m_outlinedCompositeSsaNums != nullptr)
+        {
+            m_outlinedCompositeSsaNums->Reset();
+        }
     }
 
     for (BasicBlock* const blk : Blocks())
@@ -63,13 +78,16 @@ void Compiler::fgResetForSsa()
             }
         }
 
-        for (Statement* const stmt : blk->Statements())
+        if (deepClean)
         {
-            for (GenTree* const tree : stmt->TreeList())
+            for (Statement* const stmt : blk->Statements())
             {
-                if (tree->IsAnyLocal())
+                for (GenTree* const tree : stmt->TreeList())
                 {
-                    tree->AsLclVarCommon()->SetSsaNum(SsaConfig::RESERVED_SSA_NUM);
+                    if (tree->IsAnyLocal())
+                    {
+                        tree->AsLclVarCommon()->SetSsaNum(SsaConfig::RESERVED_SSA_NUM);
+                    }
                 }
             }
         }