[DAG] Convert foldMaskedMerge to SDPatternMatch to match (m & x) | (~m & y)

[RKSimon]

Now that foldMaskedMerge has been moved to DAGCombine in #137641, it can be refactored to use SDPatternMatch matchers.

This should allow us to remove foldMaskedMergeImpl entirely and not match all the commutation permutations separately but rely on the commutative SDPatternMatch m_Or/m_And/m_Not/m_Deferred matchers to find the 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:

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3. Fix the issue locally.
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[llvmbot]

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

Author: Simon Pilgrim (RKSimon)

Now that foldMaskedMerge has been moved to DAGCombine in #137641, it can be refactored to use SDPatternMatch matchers.

This should allow us to remove foldMaskedMergeImpl entirely and not match all the commutation permutations separately but rely on the commutative SDPatternMatch m_Or/m_And/m_Not/m_Deferred matchers to find the pattern.

[woruyu]

hello, @RKSimon I would like to work for this issue! It's my first time to commit. Any advice or suggestion.

[RKSimon]

I'd start by ensuring you have a ninja build up and running - your changes must ensure check-llvm-codegen is still clean

Then review other uses of sd_match in DAGCombiner

[woruyu]

Added pr