[dataflow] Add/use bitmap_set_range

[Paolo Bonzini]

This is a simple low-hanging fruit.  Instead of using
the relatively complex df_ref_bitmap mechanism (which
builds bitmaps with a single range of 1's) we can use
those lines of code to write a new bitmap_set_range
function (to complement the existing bitmap_clear_range).

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).

I figure a reason for this could be either a different
implementation in the past, or preparing for a different
implementation in the future.

Anyway, svn is there to find the code if needed.  Ok?

Paolo
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