Since N2 will be reused in the fold, we cannot skip N2's undef elements
if the corresponding element in N1 is well-defined.
For example:
```
t2: v4i32 = BUILD_VECTOR Constant:i32<0>, Constant:i32<0>, Constant:i32<0>, Constant:i32<0>
t24: v4i32 = BUILD_VECTOR undef:i32, undef:i32, Constant:i32<1>, undef:i32
t11: v4i32 = vselect t8, t2, t10
```
Before this patch, we fold t11 into:
```
t26: v4i32 = sign_extend t8
t27: v4i32 = add t26, t24
```
The last element of t27 is incorrect.
Closes https://github.com/llvm/llvm-project/issues/129181.
2709366f75