Continue to work outlined in D155747 and split the main SPIR-V ops
implementation file into a few smaller and quicker to compile files.
Move control flow and memory ops to their own implementation files.
Create new `.cpp` files for tablegened code.
After this change, the `SPIRVOps.cpp` is 2k LoC-long and takes a
reasonable amount of time to compile.
Reviewed By: antiagainst
Differential Revision: https://reviews.llvm.org/D155883
