[spirv] Add cast operations

This CL added op definitions for a few cast operations:

* OpConvertFToU
* OpConvertFToS
* OpConvertSToF
* OpConvertUToF
* OpUConvert
* OpSConvert
* OpFConvert

Also moved the definition of spv.Bitcast to the new file.

Closes tensorflow/mlir#208 and tensorflow/mlir#174

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/208 from denis0x0D:sandbox/cast_ops 79bc9b37398aafddee6cf6beb301807988fe67f9
PiperOrigin-RevId: 277587891