JIT: Use any reaching def for unreachable uses in incremental SSA builder (#110077)

There really is no correct reaching definition in this case, so any reaching
definition will do here. This matches what happens normally in the JIT when
various phases optimize control flow after SSA has been built without removing
the now-unreachable blocks.

Also give unreachable stores an SSA number.

Fix #109971

Author: jakobbotsch

src/coreclr/jit/optcse.cpp

@@ -5114,10 +5114,7 @@ void CSE_HeuristicCommon::ReplaceCSENode(Statement* stmt, GenTree* exp, GenTree*
 //
 void CSE_HeuristicCommon::InsertUseIntoSsa(IncrementalSsaBuilder& ssaBuilder, const UseDefLocation& useDefLoc)
 {
-    if (!ssaBuilder.InsertUse(useDefLoc))
-    {
-        return;
-    }
+    ssaBuilder.InsertUse(useDefLoc);
 
     GenTreeLclVar* lcl = useDefLoc.Tree;
     assert(lcl->HasSsaName());

src/coreclr/jit/ssabuilder.cpp

@@ -1737,13 +1737,11 @@ bool IncrementalSsaBuilder::FinalizeDefs()
     for (int i = 0; i < m_defs.Height(); i++)
     {
         UseDefLocation& def = m_defs.BottomRef(i);
-        if (!m_comp->m_dfsTree->Contains(def.Block))
+        if (m_comp->m_dfsTree->Contains(def.Block))
         {
-            continue;
+            BitVecOps::AddElemD(&m_poTraits, m_defBlocks, def.Block->bbPostorderNum);
         }
 
-        BitVecOps::AddElemD(&m_poTraits, m_defBlocks, def.Block->bbPostorderNum);
-
         unsigned ssaNum = dsc->lvPerSsaData.AllocSsaNum(m_comp->getAllocator(CMK_SSA), def.Block, def.Tree);
         def.Tree->SetSsaNum(ssaNum);
         LclSsaVarDsc* ssaDsc = dsc->GetPerSsaData(ssaNum);
@@ -1763,17 +1761,13 @@ bool IncrementalSsaBuilder::FinalizeDefs()
 // Parameters:
 //   use - Location of the use
 //
-// Returns:
-//   True if the use was in a reachable block and thus has a reaching def;
-//   otherwise false.
-//
 // Remarks:
 //   All uses are required to never read an uninitialized value of the local.
 //   That is, this function requires that all paths through the function go
 //   through one of the defs in "defs" before any use in "uses" for uses that
 //   are statically reachable.
 //
-bool IncrementalSsaBuilder::InsertUse(const UseDefLocation& use)
+void IncrementalSsaBuilder::InsertUse(const UseDefLocation& use)
 {
     assert(m_finalizedDefs);
 
@@ -1788,11 +1782,14 @@ bool IncrementalSsaBuilder::InsertUse(const UseDefLocation& use)
     {
         if (!m_comp->m_dfsTree->Contains(use.Block))
         {
-            JITDUMP("  Use is in unreachable block " FMT_BB "\n", use.Block->bbNum);
-            return false;
+            reachingDef = m_defs.Bottom(0);
+            JITDUMP("  Use is in unreachable block " FMT_BB ", using first def [%06u] in " FMT_BB "\n",
+                    use.Block->bbNum, Compiler::dspTreeID(reachingDef.Tree), reachingDef.Block->bbNum);
+        }
+        else
+        {
+            reachingDef = FindOrCreateReachingDef(use);
         }
-
-        reachingDef = FindOrCreateReachingDef(use);
     }
 
     JITDUMP("  Reaching def is [%06u] d:%d\n", Compiler::dspTreeID(reachingDef.Tree), reachingDef.Tree->GetSsaNum());
@@ -1802,5 +1799,4 @@ bool IncrementalSsaBuilder::InsertUse(const UseDefLocation& use)
 
     LclVarDsc* dsc = m_comp->lvaGetDesc(m_lclNum);
     dsc->GetPerSsaData(reachingDef.Tree->GetSsaNum())->AddUse(use.Block);
-    return true;
 }

src/coreclr/jit/ssabuilder.h

@@ -151,5 +151,5 @@ class IncrementalSsaBuilder
 
     void InsertDef(const UseDefLocation& def);
     bool FinalizeDefs();
-    bool InsertUse(const UseDefLocation& use);
+    void InsertUse(const UseDefLocation& use);
 };