Add an assertion to ensure freezeReservedRegs() is only ever called once.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168633 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineRegisterInfo.cpp

@@ -305,6 +305,8 @@ void MachineRegisterInfo::dumpUses(unsigned Reg) const {
 #endif
 
 void MachineRegisterInfo::freezeReservedRegs(const MachineFunction &MF) {
+  assert (!reservedRegsFrozen() &&
+          "freezeReservedRegs should only be called once!");
   ReservedRegs = TRI->getReservedRegs(MF);
   assert(ReservedRegs.size() == TRI->getNumRegs() &&
          "Invalid ReservedRegs vector from target");