Use the advanceToLowerBound operation available on CoalescingBitVector
iterators to speed up collection of variables which reside within some
set of registers.
The speedup comes from avoiding repeated top-down traversals in
IntervalMap::find. The linear scan forward from one register interval to
the next is unlikely to be as expensive as a full IntervalMap search
starting from the root.
This reduces time spent in LiveDebugValues when compiling sqlite3 by
200ms (about 0.1% - 0.2% of the total User Time).
Depends on D76466.