[AArch64][GlobalISel] Emit G_ASSERT_ZEXT in assignValueToAddress for ZExt params

When we have a zeroext parameter coming in on the stack, build

```
%x = G_LOAD ...
%x_assert_zext = G_ASSERT_ZEXT %x, narrow_size
%trunc = G_TRUNC %x_assert_zext
```

Rather than just loading into the truncated type.

This allows us to optimize cases like this: https://godbolt.org/z/vfjhW8

Differential Revision: https://reviews.llvm.org/D95805