Add a LiveRangeUpdater class.

Adding new segments to large LiveIntervals can be expensive because the
LiveRange objects after the insertion point may need to be moved left or
right. This can cause quadratic behavior when adding a large number of
segments to a live range.

The LiveRangeUpdater class allows the LIveInterval to be in a temporary
invalid state while segments are being added. It maintains an internal
gap in the LiveInterval when it is shrinking, and it has a spill area
for new segments when the LiveInterval is growing.

The behavior is similar to the existing mergeIntervalRanges() function,
except it allocates less memory for the spill area, and the algorithm is
turned inside out so the loop is driven by the clients.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175644 91177308-0d34-0410-b5e6-96231b3b80d8

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