Init TritonGPU to LLVM dialect conversion (#32)

* add toLLVM pass

* update num-warps setting in mlir

test/Conversion/tritongpu_to_llvm.mlir

@@ -0,0 +1,9 @@
+// RUN: triton-opt %s -split-input-file --convert-triton-gpu-to-llvm=num-warps=8
+
+// CHECK: llvm.func @test_empty_kernel(%arg0: i64, %arg1: !llvm.ptr<f16, 1>)
+// CHECK: attributes {nvvm.maxntidx = 96 : i32}
+func @test_empty_kernel(%lb : index, %A : !tt.ptr<f16>) {
+
+  // CHECK:  llvm.return
+  return
+}