Replace isF...() LLVM API calls with the corresponding isa<...>()

[AndreyPavlenko]

The isF...() methods have been removed in the main LLVM branch: llvm/llvm-project#123326

New contributor declaration

• I am not making a trivial change, such as fixing a typo in a comment.

• I have written a PR description following these
  rules.

• I have run pre-commit run --from-ref origin/main --to-ref HEAD.

• Select one of the following.

  • I have added tests.
    • /test for lit tests
    • /unittest for C++ tests
    • /python/test for end-to-end tests
  • This PR does not need a test because it fixes build failures.

• Select one of the following.

  • I have not added any lit tests.
  • The lit tests I have added follow these best practices,
    including the "tests should be minimal" section. (Usually running Python code
    and using the instructions it generates is not minimal.)

[Jokeren]

@peterbell10 I'm not sure why there are several PRs that handling similar stuff. IIUC, we should focus on the other one instead? #5684

[AndreyPavlenko]

@peterbell10 I'm not sure why there are several PRs that handling similar stuff. IIUC, we should focus on the other one instead? #5684

Sorry, I wasn't aware of that one.

[peterbell10]

Agreed, closing as duplicate.