[SelectionDAG] Don't scalarize vector fpround sources that don't need it.

Similar to the workaround code in ScalarizeVecRes_UnaryOp, ScalarizeVecRes_SETCC
, ScalarizeVecRes_VSELECT, etc.

If we have a case like this:

```
define <1 x half> @func(<1 x float> %x) {
  %tmp = fptrunc <1 x float> %x to <1 x half>
  ret <1 x half> %tmp
}
```

On AArch64, the <1 x float> is legal. So, this will crash if we call
GetScalarizedVector on it.

Differential Revision: https://reviews.llvm.org/D98208