JIT: Use any reaching def for unreachable uses

Fix dotnet#109971

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);
@@ -1788,11 +1786,14 @@ 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());