[TargetRegisterInfo] Speed up getAllocatableSet. NFCI.

[foad]

Summary

MachineRegisterInfo caches the reserved register set that is computed by
by TargetRegisterInfo::getReservedRegs, so call into MRI to get the
reserved regs to avoid recomputing them.

In particular this speeds up AMDGPU's SIFormMemoryClauses pass because
AMDGPU has a particularly complicated reserved set that is expensive to
compute.

llvm/lib/CodeGen/TargetRegisterInfo.cpp

if (SubClass)
       getAllocatableSetForRC(MF, SubClass, Allocatable);
   } else {
     for (const TargetRegisterClass *C : regclasses())
       if (C->isAllocatable())
         getAllocatableSetForRC(MF, C, Allocatable);
   }
 
   // Mask out the reserved registers
-  BitVector Reserved = getReservedRegs(MF);
-  Allocatable &= Reserved.flip();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  const BitVector &Reserved = MRI.getReservedRegs();
+  Allocatable.reset(Reserved);
 
   return Allocatable;
 }
 
 static inline
 const TargetRegisterClass *firstCommonClass(const uint32_t *A,
                                             const uint32_t *B,
                                             const TargetRegisterInfo *TRI) {