[RISCV][ISel] Use vaaddu with rounding mode rnu for ISD::AVGCEILU. (#77473)

Similar to #76550, but for `ISD::AVGCEILU`.
Specifically, this patch aims to use `vaaddu` with rounding mode rnu
(i.e `vxrm[1:0] = 0b00`) for `ISD::AVGCEILU`.

### Source code 
```
define <vscale x 8 x i8> @vaaddu_vv_nxv8i8_ceil(<vscale x 8 x i8> %x, <vscale x 8 x i8> %y) {
  %xzv = zext <vscale x 8 x i8> %x to <vscale x 8 x i16>
  %yzv = zext <vscale x 8 x i8> %y to <vscale x 8 x i16>
  %add = add nuw nsw <vscale x 8 x i16> %xzv, %yzv
  %one = insertelement <vscale x 8 x i16> poison, i16 1, i32 0
  %splat = shufflevector <vscale x 8 x i16> %one, <vscale x 8 x i16> poison, <vscale x 8 x i32> zeroinitializer
  %add1 = add nuw nsw <vscale x 8 x i16> %add, %splat
  %div = lshr <vscale x 8 x i16> %add1, %splat
  %ret = trunc <vscale x 8 x i16> %div to <vscale x 8 x i8>
  ret <vscale x 8 x i8> %ret
}
```

### Before this patch 
```
vaaddu_vv_nxv8i8_ceil:
        vsetvli a0, zero, e8, m1, ta, ma
        vwaddu.vv       v10, v8, v9
        vsetvli zero, zero, e16, m2, ta, ma
        vadd.vi v10, v10, 1
        vsetvli zero, zero, e8, m1, ta, ma
        vnsrl.wi        v8, v10, 1
        ret
```
### After this patch 
```
vaaddu_vv_nxv8i8_ceil:
        vsetvli a0, zero, e8, m1, ta, ma
        csrwi vxrm, 0
        vaaddu.vv v8, v8, v9
        ret
```