[RISCV][Isel] Remove redundant vmerge for the scalable vwadd(u).wv (#80079)

Similar to #78403, but for scalable `vwadd(u).wv`, given that #76785 is recommited.

### Code
```
define <vscale x 8 x i64> @vwadd_wv_mask_v8i32(<vscale x 8 x i32> %x, <vscale x 8 x i64> %y) {
    %mask = icmp slt <vscale x 8 x i32> %x, shufflevector (<vscale x 8 x i32> insertelement (<vscale x 8 x i32> poison, i32 42, i64 0), <vscale x 8 x i32> poison, <vscale x 8 x i32> zeroinitializer)
    %a = select <vscale x 8 x i1> %mask, <vscale x 8 x i32> %x, <vscale x 8 x i32> zeroinitializer
    %sa = sext <vscale x 8 x i32> %a to <vscale x 8 x i64>
    %ret = add <vscale x 8 x i64> %sa, %y
    ret <vscale x 8 x i64> %ret
}
```

### Before this patch
[Compiler Explorer](https://godbolt.org/z/xsoa5xPrd)
```
vwadd_wv_mask_v8i32:
        li      a0, 42
        vsetvli a1, zero, e32, m4, ta, ma
        vmslt.vx        v0, v8, a0
        vmv.v.i v12, 0
        vmerge.vvm      v24, v12, v8, v0
        vwadd.wv        v8, v16, v24
        ret
```

### After this patch
```
vwadd_wv_mask_v8i32:
        li a0, 42
        vsetvli a1, zero, e32, m4, ta, ma
        vmslt.vx v0, v8, a0
        vsetvli zero, zero, e32, m4, tu, mu
        vwadd.wv v16, v16, v8, v0.t
        vmv8r.v v8, v16
        ret
```