[llvm] [SandboxVectorizer] Use sbvec-passes flag to create a pipeline of Region passes after BottomUpVec. (PR #111223)

[Arthur Eubanks via llvm-commits]

================
@@ -17,10 +21,22 @@ class TargetTransformInfo;
 class SandboxVectorizerPass : public PassInfoMixin<SandboxVectorizerPass> {
   TargetTransformInfo *TTI = nullptr;
 
-public:
-  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+  // Used to build a RegionPass pipeline to be run on Regions created by the
+  // bottom-up vectorization pass.
+  sandboxir::PassRegistry PR;
+
+  // The main vectorizer pass.
+  std::unique_ptr<sandboxir::BottomUpVec> BottomUpVecPass;
----------------
aeubanks wrote:

if we move away from `cl::opt("sbvec-passes")` and to proper pass parameters, we would want the LLVM pass to hold the region pass manager in the class, rather than keep it local to `runImpl`, because we'd specify the region pass pipeline via the constructor

but with `cl::opt` I don't think it really matters

https://github.com/llvm/llvm-project/pull/111223