Optimize df_worklist_dataflow

[Jan Hubicka]

>> I am tracking age of basic block.  One age is last time when BB info has
>> changed and other age is last time it was re-scanned. When scanning we need to
>> compute confluences only of those basic blocks that changed since last
>> checking.
>
> Steven is the one who experimented with all the dataflow solvers so I'll  
> gladly defer this review to him.
OK :)
Note that today profile has changed (I believe due to Eric's patch).
The magical slowness of fast_dce I intended to analyze is gone.  I guess problem
was/is that we do not fit the coniditonals for removing pure/const calls on
SSA,du and fast DCEs that might result in scenrario where fast_dce executed
after reload needs many iterations since it is first instance removing some
pure calls.  If the problem returns back, I will double check the theory but
it seems only plausible explanation.

Current profile is:
51198     3.8267  lto1                     htab_find_slot_with_hash
19428     1.4521  lto1                     df_worklist_dataflow
17117     1.2794  lto1                     bitmap_set_bit_1
16122     1.2050  lto1                     df_note_compute
14601     1.0913  lto1                     htab_traverse_noresize
12909     0.9649  lto1                     record_reg_classes.constprop.1
12652     0.9457  lto1                     htab_find_with_hash
11987     0.8960  lto1                     ggc_internal_alloc_stat
11802     0.8821  lto1                     bitmap_clear_bit
10892     0.8141  lto1                     walk_tree_1
10197     0.7622  lto1                     bitmap_copy
8886      0.6642  lto1                     bitmap_bit_p_1
8658      0.6471  lto1                     et_splay

I am wondering if there are any obvious improvements for df_note_compute?
One thing I noticed is that it might be reorganized to walk reverse superblocks
avoid copying the live bitmaps all the time (it is also one of most busy callers
of bitmap_copy).  But I guess benefits of such a trick won't be that grand
and it is bit difficult to reorganize it since it is executed at all_blocks bitmap
(that is I believe always whole CFG) and anything that tries to walk superblocks
would result in random access to it.

Honza