Output LLVM IR if NUMBA_DPEX_DEBUG environment variable is set

[adarshyoga]

• Have you provided a meaningful PR description?

Adding functionality to output LLVM IR to a file. The output file name contains a hash generated from the LLVM module name. This allows us to write different kernels to separate files since the hash is expected to be unique.

• Have you added a test, reproducer or referred to an issue with a reproducer?

Tested with Blackscholes in dpbench. File name generated is llvm_kernel_ce84b7ebc25a6c5f71403d77de797be75067dce745a14ae28ca34dc5e3365998.ll

• Have you tested your changes locally for CPU and GPU devices?
  Tested on GPU

• Have you made sure that new changes do not introduce compiler warnings?

• If this PR is a work in progress, are you filing the PR as a draft?

[diptorupd]

Can we use a new flag NUMBA_DPEX_DUMP_KERNEL_LLVM? The DEBUG seems to turn on debug symbols in some places.

[adarshyoga]

Added. Could not find any place where documentation needs to be updated with the env var. If I have missed it, let me know.

[mingjie-intel]

Tested the code with small code from #906 , llvm IR is printed.

[diptorupd]

@adarshyoga Thank you! It is good to go, but we need a test case. Just to verify that setting the config actually generates the IR file in the right location and the file is not empty.

Refer: https://docs.pytest.org/en/6.2.x/tmpdir.html#the-tmpdir-fixture

[adarshyoga]

@adarshyoga Thank you! It is good to go, but we need a test case. Just to verify that setting the config actually generates the IR file in the right location and the file is not empty.

Refer: https://docs.pytest.org/en/6.2.x/tmpdir.html#the-tmpdir-fixture

Added a test case that tests both positive and negative cases.