[DAG] SelectionDAG::canCreateUndefOrPoison - add ISD::SCMP/UCMP handling + tests

[RKSimon]

Add ISD::SCMP/UCMP to SelectionDAG::canCreateUndefOrPoison along with suitable test coverage

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor is working on this issue. If someone is assigned to the issue or claimed to be working on it, ping the person. After one week without a response, the assignee may be changed.
2. Leave a comment indicating that you are working on the issue, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Simon Pilgrim (RKSimon)

Add ISD::SCMP/UCMP to SelectionDAG::canCreateUndefOrPoison along with suitable test coverage

[Temperz87]

@RKSimon can I be assigned to this task?

[Temperz87]

Just to be clear, this task should involve:

1. Adding ISD::SCMP and ISD::UCMP to this function (and having them return true in certain INT_MAX or INT_MIN cases)
2. Writing a bunch of unit tests. Where would be a good place to put these tests?

[RKSimon]

One of the CodeGen/X86/freeze*.ll files probably makes sense. You'll probably have to use a knownbits pattern to detect when the freeze(cmp(x,y)) has folded to cmp(freeze(x),freeze(y))