[Mips] Reserve hardware register HWR2.

Fix pr #127553.
x86_64 failed to run readcyclecounter.ll when enable expensive_check,
it would error "Using an undefined physical register".

Author: yingopq

llvm/lib/Target/Mips/MipsRegisterInfo.cpp

@@ -205,6 +205,7 @@ getReservedRegs(const MachineFunction &MF) const {
 
   // Reserve hardware registers.
   Reserved.set(Mips::HWR29);
+  Reserved.set(Mips::HWR2);
 
   // Reserve DSP control register.
   Reserved.set(Mips::DSPPos);