[AArch64][GlobalISel] Widen s2 and s4 G_IMPLICIT_DEF + G_FREEZE

These had

```
.clampScalar(0, s1, 64)
.widenScalarToNextPow2(0, 8)
```

If you have s2 or s4, then `widenScalarToNextPow2` does nothing.

This changes the `widenScalarToNextPow2` rule to use s8 as the minimum type
instead, allowing us to correctly widen s2 and s4.

This does not impact s1, since it's marked as legal already.

Differential Revision: https://reviews.llvm.org/D106413