[CodeGen] Remove an unnecessary cast (NFC) (#148764)

getExpression() already returns DIExpression *.

Author: kazutakahirata

llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp

@@ -784,7 +784,7 @@ MachineInstr *
 InstrEmitter::EmitDbgInstrRef(SDDbgValue *SD,
                               VRBaseMapType &VRBaseMap) {
   MDNode *Var = SD->getVariable();
-  const DIExpression *Expr = (DIExpression *)SD->getExpression();
+  const DIExpression *Expr = SD->getExpression();
   DebugLoc DL = SD->getDebugLoc();
   const MCInstrDesc &RefII = TII->get(TargetOpcode::DBG_INSTR_REF);