[LiveDebugValues] Speed up collectIDsForRegs, NFC

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.

rdar://60046261

Differential Revision: https://reviews.llvm.org/D76467