[TargetRegisterInfo] Speed up getAllocatableSet. NFCI.

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.

Differential Revision: https://reviews.llvm.org/D102318