Also call optimize_node() instead of only calling transform_node_*() or doing nothing.

This time care for used constbits.

This fixes opt/fehler234.c.

This reverts commit 59435e3b.
Counterexample: (x & 0xFF00) + 0xFF, with a 16-bit x.

This partially reverts commit 59435e3b.

We may propagate don't care information for bits that
we used to infer other don't care information.

This reverts commit 26e639c4.
We cannot benefit from bitinfo if we want *exactly* one bit.

- Managed resources within the analysis itself
- Let constbits take care of the obstack to store the bit information

Mux((a & (1 << c)) != (b & (1 << c)), 0, 1) => (a ^ b) >> c.

Didn't notice this before because gcc doesn't report a warning for this,
clang does.