[LiveIntervals] Find better anchoring end points when repairing ranges

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