[SelectionDAG] Add m_ZExtOrSelf/m_SExtOrSelf/m_AExtOrSelf/m_TruncOrSelf pattern matchers

[RKSimon]

Followup to #85065

Similar to what we have in PatternMatch.h - allows us to peek though optional extensions/truncations

CC @mshockwave

[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. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. 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.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

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)

Followup to #85065

Similar to what we have in PatternMatch.h - allows us to peek though optional extensions/truncations

CC @mshockwave

[marcauberer]

@RKSimon I'd like to work on this