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

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -313,12 +306,14 @@ class SPIRVStructurizerImpl {
     LoopInfo &LI;
     DomTreeBuilder::BBDomTree DT;
     DomTreeBuilder::BBPostDomTree PDT;
+    std::optional<PartialOrderingVisitor> POV;
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jmmartinez wrote:

If I'm not wrong, `PartialOrderingVisitor` is declared/defined in `SPIRVUtils.h/.cpp`, but only used it by this pass (and both, this pass and the visitor look... scrappy). What do you think about moving it to its own header/source to isolate it?

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