[llvm] Add SD matchers and unit test coverage for ISD::VECTOR_SHUFFLE (PR #119592)

[Aidan Goldfarb via llvm-commits]

AidanGoldfarb wrote:

I just pushed some changes that mirror the PatternMatch approach as closely as possible. This includes
`m_Mask` and `m_SpecificMask`, with room to add things like `m_ZeroMask`. I did not add `m_ZeroMask` or `m_SplatOrPoisonMask` but would be happy to do it if there is a use case. 


https://github.com/llvm/llvm-project/pull/119592