Wrong RTL instruction deleted

[Florian Weimer]

* Robert Dewar:

> Presumably the Ada program would fail in the same way.

If Unchecked_Conversion is involved, the front end should provide
correct alias sets.  In this case, the result should be a
VIEW_CONVERT_EXPR (if I read the sources correctly), and it hopefully
does the right thing.

Using -fno-strict-aliasing throws away a lot of the additional
information the Ada front end can provide, compared to equivalent C
code. 8-(

Hmm, I think VIEW_CONVERT_EXPR could be a way to implement the
unspecified behavior in 5.2.10(9, 10) in a more deterministic manner.