[mlir][Vector] Mostly-NFC - Restructure options for lowering to LLVM Matrix Intrinsics

Summary:
This revision restructures the calling of vector transforms to make it more flexible to ask for lowering through LLVM matrix intrinsics.
This also makes sure we bail out in degenerate cases (i.e. 1) in which LLVM complains about not being able to scalarize.

Differential Revision: https://reviews.llvm.org/D76266

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -1150,8 +1150,8 @@ void LowerVectorToLLVMPass::runOnModule() {
   }
 }
 
-OpPassBase<ModuleOp> *mlir::createLowerVectorToLLVMPass() {
-  return new LowerVectorToLLVMPass();
+std::unique_ptr<OpPassBase<ModuleOp>> mlir::createConvertVectorToLLVMPass() {
+  return std::make_unique<LowerVectorToLLVMPass>();
 }
 
 static PassRegistration<LowerVectorToLLVMPass>