JIT: fix def tree for CSE locals going into SSA (dotnet/coreclr#27301)

When putting a new single-def CSE local into SSA, we need to set the def tree
to the LHS of the GT_ASG, not the GT_ASG.

Commit migrated from dotnet/coreclr@90e59be

src/coreclr/src/jit/optcse.cpp

@@ -2381,7 +2381,7 @@ class CSE_Heuristic
 
                     ssaVarDsc->m_vnPair        = val->gtVNPair;
                     ssaVarDsc->m_defLoc.m_blk  = blk;
-                    ssaVarDsc->m_defLoc.m_tree = asg;
+                    ssaVarDsc->m_defLoc.m_tree = asg->AsOp()->gtOp1;
                 }
 
                 /* Create a reference to the CSE temp */