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

[Arseniy Obolenskiy via llvm-commits]

aobolensk wrote:

@jmmartinez suggestions LGTM, but should I address your comments in the separate PR or here?

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