[llvm] [SDPatternMatch] Add Matcher m_Undef (PR #122521)

[Jon Roelofs via llvm-commits]

jroelofs wrote:

@RKSimon can you suggest a pattern to use this with? I think it would be better to land this with a real use, rather than having it _only_ be used by the UnitTest.

https://github.com/llvm/llvm-project/pull/122521