[DAG] SDPatternMatch - add generic m_TernaryOp() / m_c_TernaryOp() matchers

[RKSimon]

Similar to m_BinOp / m_c_BinOp - to allow us to match a fixed opcode with 3 operands (fma ops etc.).

This mainly involves adding a generic wrapper for the TernaryOpc_match template along with suitable test coverage in SelectionDAGPatternMatchTest.cpp - there might be some simple combines in DAGCombiner.cpp that can be converted as well.

[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:

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3. Fix the issue locally.
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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)

Similar to m_BinOp / m_c_BinOp - to allow us to match a fixed opcode with 3 operands (fma ops etc.).

This mainly involves adding a generic wrapper for the TernaryOpc_match template along with suitable test coverage in SelectionDAGPatternMatchTest.cpp - there might be some simple combines in DAGCombiner.cpp that can be converted as well.

[Michael-Chen-NJU]

Can I give it a try?