[mlir][spirv] Extract Atomic/Cast/Group op implementation. NFC.

Continue to work outlined in D155747 and split the main SPIR-V ops
implementation file into a few smaller and quicker to compile files.
This organization matches the op definition organizaion in `.td` files.

In this patch, extract atomic, cast/conversion, and group op
implementation into separate files.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D155777