[BACKEND][CPU] Convert tt.func and tt.return

[minjang]

Summary: This is stll a kind of the boilerplate and basic lowering for the first milestone (compiling vector addition). This PR firstly lowers tt.func and tt.return.

Test Plan: It can safely compile an empty kernel.

@triton.jit
def add_kernel(x_ptr,  y_ptr, output_ptr, n_elements, BLOCK_SIZE: tl.constexpr):
    return

> TRITON_ENABLE_LLVM_DEBUG=1 TRITON_CPU_BACKEND=1 python3 empty_kernel.py
...
//===-------------------------------------------===//
Legalizing operation : 'tt.func'(0x73be2a0) {
  * Fold {
  } -> FAILURE : unable to fold

  * Pattern : 'tt.func -> ()' {
Trying to match "(anonymous namespace)::FuncOpConversion"
    ** Insert  : 'llvm.func'(0x6c04c70)
    ** Insert Block into : 'llvm.func'(0x6c04c70)
    ** Insert Block into : 'llvm.func'(0x6c04c70)
    ** Erase   : 'tt.func'(0x73be2a0)
"(anonymous namespace)::FuncOpConversion" result 1

    //===-------------------------------------------===//
    Legalizing operation : 'llvm.func'(0x6c04c70) {
    } -> SUCCESS : operation marked legal by the target
    //===-------------------------------------------===//

...

//===-------------------------------------------===//
Legalizing operation : 'tt.return'(0x73efeb0) {
  "tt.return"() : () -> ()

  * Fold {
  } -> FAILURE : unable to fold

  * Pattern : 'tt.return -> ()' {
Trying to match "(anonymous namespace)::ReturnOpConversion"
    ** Insert  : 'llvm.return'(0x73c0f00)
    ** Replace : 'tt.return'(0x73efeb0)
"(anonymous namespace)::ReturnOpConversion" result 1

    //===-------------------------------------------===//
    Legalizing operation : 'llvm.return'(0x73c0f00) {
      "llvm.return"() : () -> ()

    } -> SUCCESS : operation marked legal by the target
    //===-------------------------------------------===//
  } -> SUCCESS : pattern applied successfully

...


INSTCOMBINE ITERATION #1 on add_kernel
ADD:   ret void, !dbg !6
IC: Visiting:   ret void, !dbg !6
tensor([1.3713, 1.3076, 0.4940,  ..., 0.6724, 1.2141, 0.9733], device='cuda:0')
tensor([0., 0., 0.,  ..., 0., 0., 0.], device='cuda:0')
The maximum difference between torch and triton is 1.9935195446014404