[AMDGPU] Replace getPhysRegClass with getPhysRegBaseClass

Accelerate finding the base class for a physical register by
building a statically mapping table from physical registers
to base classes using TableGen.

Replace uses of SIRegisterInfo::getPhysRegClass with
TargetRegisterInfo::getPhysRegBaseClass in order to use
the computed table.

Reviewed By: arsenm, foad

Differential Revision: https://reviews.llvm.org/D139422