[DAG] SDPatternMatch - add m_Undef matcher

[RKSimon]

Add SDPatternMatch matcher and unit test coverage for ISD::UNDEF opcode

e.g.

m_InsertSubVector(m_Undef(), m_Value(), m_Zero()) // subvector widening pattern

[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 has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, 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 SDPatternMatch matcher and unit test coverage for ISD::UNDEF opcode

e.g.

m_InsertSubVector(m_Undef(), m_Value(), m_Zero()) // subvector widening pattern

[AmrDeveloper]

Interested, I can work on it