[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;
 
     Splitter(Function &F, LoopInfo &LI) : F(F), LI(LI) { invalidate(); }
 
     void invalidate() {
       PDT.recalculate(F);
       DT.recalculate(F);
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jmmartinez wrote:

If you want to go deeper, `PartialOrderingVisitor` also has a `DominatorTree`. Maybe we can reuse that one and stop recomputing 2 dominator trees.

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