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

…ns (llvm#117582)

* 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