We often recompute aliases even when absolutely nothing has changed.
This is quite wasteful.
The attached changes compute_may_aliases into a TODO, and we now only
run it when we've changed things that could destroy aliases.

Well, almost.  Everywhere but PRE this is true.  We still run aliasing
all the time after PRE, mainly because i haven't gotten the condition
quite right in PRE yet, so i left it out of the patch.

I also completely removed the may_alias after DSE, it wasn't clear
what this was trying to do or why it was there.

Sadly, the one annoying part is that there is now no easy way to put
all the aliasing stuff into their own numbered dumps.   For 4.4, i'd
really like to redo our pass manager to support this (and maybe
support a better notion of analysis, so that individual passes that
currently calculate/free dominators on their own, would just say "i
need dominators, i destroy dominators", and it is taken care of for
them).  The dump file part shouldn't be that much work, we just always
compute the pass number rather than only once per function.  We also
then track how many times we have seen a thing named "whatever", and
come up with a dump name that way.

I changed the tests looking in .alias1, to look in .salias, which is
where alias1 is now really going.

--Dan

Bootstrapped and regtested on i686-darwin.
Okay for mainline?
2007-08-10  Daniel Berlin  <dberlin@dberlin.org>

	* tree-pass.h (PROP_pta): Removed.
	(TODO_rebuild_alias): New.
	(pass_may_alias): Removed.
	* tree-ssa-ccp.c (execute_fold_all_builtins): Only rebuild
	aliasing if we changed something.
	* tree-ssa-alias.c (compute_may_aliases): Make non-static.  Update
	SSA internally.
	(pass_may_alias): Removed.
	(create_structure_vars): Return TODO_rebuild_alias.
	* tree-ssa-pre.c (do_pre): Return TODO_rebuild_alias.
	* tree-sra.c (tree_sra): Only rebuild aliasing if something
	changed.
	(tree_sra_early): We never affect aliasing right now.
	* tree-flow.h (compute_may_aliases): New prototype.
	* passes.c: Remove pass_may_alias from the passes.
	(execute_function_todo): Support TODO_rebuild_alias.