[NFC] Add the getSizeInBytes() interface for MachineConstantPoolValue

Current implementation assumes that, each MachineConstantPoolValue takes
up sizeof(MachineConstantPoolValue::Ty) bytes. For PowerPC, we want to
lump all the constants with the same type as one MachineConstantPoolValue
to save the cost that calculate the TOC entry for each const. So, we need
to extend the MachineConstantPoolValue that break this assumption.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D89108