[DAGCombiner] Don't fold FCOPYSIGN vector sign operand casts

Avoid doing the following combine for vector types:

```
copysign(x, fp_extend(y)) -> copysign(x, y)
copysign(x, fp_round(y)) -> copysign(x, y)
```

That combine seemed to impede the selection of vector instruction and cause
a mess in some circumstances.

Differential Revision: https://reviews.llvm.org/D96037