[llvm-branch-commits] [DirectX] Encapsulate DXILOpLowering's state into a class. NFC (PR #104248)

[Justin Bogner via llvm-branch-commits]

https://github.com/bogner created https://github.com/llvm/llvm-project/pull/104248

This introduces an anonymous class "OpLowerer" to help with lowering DXIL ops,
and moves the DXILOpBuilder there instead of creating a new one for every
operation. DXILOpBuilder is also changed to own its IRBuilder, since that makes
it simpler to ensure that it isn't misused.