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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@AidanGoldfarb Do you think we could keep closer to the PatternMatch approach of m_Shuffle / m_Mask / m_SpecificMask (and m_SplatOrPoisonMask / m_ZeroMask .....) ?

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