Merged two loops that were iterating over the same machine basic block
into one, also did some minor readability improvements (variable
renaming, commenting and absorbing if condition into a variable)

- Add `SlotIndexesAnalysis`.
- Add `SlotIndexesPrinterPass`.
- Use `SlotIndexesWrapperPass` in legacy pass.

simple_ilist does not take ownership of its nodes, which is fine for
SlotIndexes because the IndexListEntry nodes are allocated with a
BumpPtrAllocator and do not need to be freed.

This reverts commit 2501ae58e3bb9a70d279a56d7b3a0ed70a8a852c.

Reverted due to various buildbot failures.

PR #66334 tried to renumber slot indexes before register allocation, but
the numbering was still affected by list entries for instructions which
had been erased. Fix this to make the register allocator's live range
length heuristics even less dependent on the history of how instructions
have been added to and removed from SlotIndexes's maps.

RegAllocGreedy uses SlotIndexes::getApproxInstrDistance to approximate
the length of a live range for its heuristics. Renumbering all slot
indexes with the default instruction distance ensures that this estimate
will be as accurate as possible, and will not depend on the history of
how instructions have been added to and removed from SlotIndexes's maps.

This also means that enabling -early-live-intervals, which runs the
SlotIndexes analysis earlier, will not cause large amounts of churn due
to different register allocator decisions.

r175673 changed repairIntervalsInRange to find anchoring end points for
ranges automatically, but the calculation of Begin included the first
instruction found that already had an index. This patch changes it to
exclude that instruction:

1. For symmetry, so that the half open range [Begin,End) only includes
   instructions that do not already have indexes.
2. As a possible performance improvement, since repairOldRegInRange
   will scan fewer instructions.
3. Because repairOldRegInRange hits assertion failures in some cases
   when it sees a def that already has a live interval.

(3) fixes about ten tests in the CodeGen lit test suite when
-early-live-intervals is forced on.

Differential Revision: https://reviews.llvm.org/D110182

Identified with modernize-use-default-member-init.