[DebugInfo][InstrRef] Track a single variable at a time

Here's another performance patch for InstrRefBasedLDV: rather than
processing all variable values in a scope at a time, instead, process one
variable at a time. The benefits are twofold:
 * It's easier to reason about one variable at a time in your mind,
 * It improves performance, apparently from increased locality.

The downside is that the value-propagation code gets indented one level
further, plus there's some churn in the unit tests.

Differential Revision: https://reviews.llvm.org/D111799