[dataflow] Add/use bitmap_set_range

[Paolo Bonzini]

>> This is compile-time neutral on bootstrap, saves 2%
>> on PR28071 -O2.  Might save a few megabytes of memory,
>> though not much (by the way: it might be time for another
>> merge; there were some improvements on mainline lately
>> with respect to memory savings).
>>
> Did you measure the space, the reason for using the mechanism that I did
> was a space issue.  However, if it is neutral or even close, put it in.

I'm not killing the sparse/dense mechanism, just replacing the
df_ref_bitmap cache with calls to bitmap_set_range (when it was ORed)
and bitmap_clear_range (when it was AND_COMPLed).

Paolo