Interpreter: Remove an unnecessary UReg_MSR& cast from SingleStepInner

This is technically undefined behavior, but regardless of that, it's not
even necessary since we can just make a temporary around the MSR value
and just discard it when done with it, since all we do is query the FP
bit value with it.

Source/Core/Core/PowerPC/Interpreter/Interpreter.cpp

@@ -152,7 +152,7 @@ int Interpreter::SingleStepInner()
 
     if (m_prev_inst.hex != 0)
     {
-      UReg_MSR& msr = (UReg_MSR&)MSR;
+      const UReg_MSR msr{MSR};
       if (msr.FP)  // If FPU is enabled, just execute
       {
         m_op_table[m_prev_inst.OPCD](m_prev_inst);