Make sure the whole live range is covered when values are pruned twice.

JoinVals::pruneValues() calls LIS->pruneValue() to avoid conflicts when
overlapping two different values. This produces a set of live range end
points that are used to reconstruct the live range (with SSA update)
after joining the two registers.

When a value is pruned twice, the set of end points was insufficient:

  v1 = DEF
  v1 = REPLACE1
  v1 = REPLACE2
  KILL v1

The end point at KILL would only reconstruct the live range from
REPLACE2 to KILL, leaving the range REPLACE1-REPLACE2 dead.

Add REPLACE2 as an end point in this case so the full live range is
reconstructed.

This fixes PR13999.

llvm-svn: 165056