[RFC] Remove first_pass_instance from pass_vrp

[Tom de Vries]

Hi,

[ See also related discussion at 
https://gcc.gnu.org/ml/gcc-patches/2012-07/msg00452.html ]

this patch removes the usage of first_pass_instance from pass_vrp.

the patch:
- limits itself to pass_vrp, but my intention is to remove all
   usage of first_pass_instance
- lacks an update to gdbhooks.py

Modifying the pass behaviour depending on the instance number, as 
first_pass_instance does, break compositionality of the pass list. In 
other words, adding a pass instance in a pass list may change the 
behaviour of another instance of that pass in the pass list. Which 
obviously makes it harder to understand and change the pass list. [ I've 
filed this issue as PR68247 - Remove pass_first_instance ]

The solution is to make the difference in behaviour explicit in the pass 
list, and no longer change behaviour depending on instance number.

One obvious possible fix is to create a duplicate pass with a different 
name, say 'pass_vrp_warn_array_bounds':
...
   NEXT_PASS (pass_vrp_warn_array_bounds);
   ...
   NEXT_PASS (pass_vrp);
...

But, AFAIU that requires us to choose a different dump-file name for 
each pass. And choosing vrp1 and vrp2 as new dump-file names still means 
that -fdump-tree-vrp no longer works (which was mentioned as drawback 
here: https://gcc.gnu.org/ml/gcc-patches/2012-07/msg00453.html ).

This patch instead makes pass creation parameterizable. So in the pass 
list, we use:
...
   NEXT_PASS_WITH_ARG (pass_vrp, true /* warn_array_bounds_p */);
   ...
   NEXT_PASS_WITH_ARG (pass_vrp, false /* warn_array_bounds_p */);
...

This approach gives us clarity in the pass list, similar to using a 
duplicate pass 'pass_vrp_warn_array_bounds'.

But it also means -fdump-tree-vrp still works as before.

Good idea? Other comments?

Thanks,
- Tom
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