[Transforms] Fix -Wunused-function for 'GetReplicateRegion' with -DLLVM_ENABLE_ASSERTIONS=OFF (NFC)

/Users/jiefu/llvm-project/llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp:614:23: error: unused function 'GetReplicateRegion' [-Werror,-Wunused-function]
static VPRegionBlock *GetReplicateRegion(VPRecipeBase *R) {
                      ^
1 error generated.