[llvm] [SPIR-V] Cache PartialOrderingVisitor in Splitter instead of rebuilding per call (PR #211198)

[Juan Manuel Martinez Caamaño via llvm-commits]

https://github.com/jmmartinez approved this pull request.

The change looks good to me but for me to have the context: are you checking the structurizer for anything in particular ?

https://github.com/llvm/llvm-project/pull/211198