[improved-aliasing]: Remove possible O(n^2) behavior in call clobbering

[Daniel Berlin]

Right now we do things like:

          if (pi->pt_vars)
            EXECUTE_IF_SET_IN_BITMAP (pi->pt_vars, 0, j, bi)
              mark_call_clobbered (referenced_var (j));

mark_call_clobbered, in turn, is:


/* Mark variable VAR as being clobbered by function calls.  */
static inline void
mark_call_clobbered (tree var)
{
  bitmap_set_bit (call_clobbered_vars, DECL_UID (var));
  ssa_call_clobbered_cache_valid = false;
  ssa_ro_call_cache_valid = false;
}

Note we do a linear iteration (bitmap walk) -> hashtable lookup
(referenced var lookup) -> bit find and set (bitmap_set_bit).

All to do a bitmap union.

if we had a 

static inline void
mark_bitmap_call_clobbered (bitmap ids)
{
  bitmap_ior_into (call_clobbered_vars, ids);
  ssa_call_clobbered_cache_valid = false;
  ssa_ro_call_cache_valid = false;
}


You could then just replace it with
if (pt->pt_vars)  
  mark_bitmap_call_clobbered (pi->pt_vars)


Which is what i've done on improved-aliasing :)
One could apply the same trick to the mainline, if they felt so
inclined.

Bootstrapped and regtested on i686-pc-linux-gnu.
Applied to improved-aliasing-branch
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