[SelectOpt] Refactor to prepare for support more select-like operations

* Enables conversion of several select-like instructions within one group
* Any number of auxiliary instructions depending on the same condition can be
  in between select-like instructions
* After splitting the basic block, move select-like instructions into the
  relevant basic blocks and optimise them
* Make it easier to add support shift-base select-like instructions and also
  any mixture of zext/sext/not instructions

Author: igogo-x86