constified note_stores

[Ian Lance Taylor]

Joern Rennecke <joernr@arc.com> writes:

> Having another copy of note_stores seems simpler and is certainly
> more portable.
> What do you think about the name walk_stores?

Following this approach strictly leads to considerable code
duplication, which makes people unhappy.  If you want to make an
argument for just using it for note_stores, I'm willing to listen.  I
think the new name should be tied to note_stores, though--e.g.,
note_stores_nonconst.

It's definitely a problem in C that some functions reasonably take
const pointers in some cases and non-const pointers in other cases.
But I think it's a well understood problem, and I think
CONST_CAST_RTX, or small structures, are acceptable approaches.
Obviously if we had to use CONST_CAST_RTX everywhere something would
be wrong, but I only count six uses in the core code right now, which
seems like an acceptable compromise.

Ian