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

[Min-Yih Hsu via llvm-commits]

================
@@ -226,6 +226,15 @@ inline SwitchContext<MatchContext, Pattern> m_Context(const MatchContext &Ctx,
   return SwitchContext<MatchContext, Pattern>{Ctx, std::move(P)};
 }
 
+struct m_Mask {
----------------
mshockwave wrote:

this is not used anymore I believe

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