[llvm] [DirectX] Propagate shader flags mask of callees to callers (PR #118306)

[S. Bharadwaj Yadavalli via llvm-commits]

bharadwajy wrote:

> This looks fine, though the propagate-function-flags test is pretty inscrutable. It would be nice to use function names that make it clearer what's being tested, and probably have some comments about what the CFG looks like, otherwise this test will be more or less impossible to update in the future.

Updated the test as suggested. Thanks!

https://github.com/llvm/llvm-project/pull/118306