Add convergent as a supported FunctionAttribute and CallInstrAttribute.

[diptorupd]

The PR adds a new attribute string to the set of known FunctionAttributes and CallInstrAttributes.

The convergent attribute is needed for certain function declarations and call instructions to generate correct SPMD code for OpenCL. Refer: https://llvm.org/docs/ConvergentOperations.html

[stuartarchibald]

Thanks for the patch. This seems like an appropriate addition to support IR generation for convergent operation semantics. It would be good to add a check for IR generation into a suitable existing tests in llvmlite/tests/test_ir.py, thank you.

[diptorupd]

Thanks for the patch. This seems like an appropriate addition to support IR generation for convergent operation semantics. It would be good to add a check for IR generation into a suitable existing tests in llvmlite/tests/test_ir.py, thank you.

@stuartarchibald I have added some basic unit tests. Please let me know if these are adequate.

[stuartarchibald]

Thanks for adding the tests @diptorupd, one minor comment to resolve else looks good.

[diptorupd]

Closing this PR in favour of #1019