[LiveIntervalAnalysis] Speed up creation of live ranges for physical registers

by using a segment set.

The patch addresses a compile-time performance regression in the LiveIntervals
analysis pass (see http://llvm.org/bugs/show_bug.cgi?id=18580). This regression
is especially critical when compiling long functions. Our analysis had shown
that the most of time is taken for generation of live intervals for physical
registers. Insertions in the middle of the array of live ranges cause quadratic
algorithmic complexity, which is apparently the main reason for the slow-down.

Overview of changes:
- The patch introduces an additional std::set<Segment>* member in LiveRange for
  storing segments in the phase of initial creation. The set is used if this
  member is not NULL, otherwise everything works the old way.
- The set of operations on LiveRange used during initial creation (i.e. used by
  createDeadDefs and extendToUses) have been reimplemented to use the segment
  set if it is available.
- After a live range is created the contents of the set are flushed to the
  segment vector, because the set is not as efficient as the vector for the
  later uses of the live range. After the flushing, the set is deleted and
  cannot be used again.
- The set is only for live ranges computed in
  LiveIntervalAnalysis::computeLiveInRegUnits() and getRegUnit() but not in
  computeVirtRegs(), because I did not bring any performance benefits to
  computeVirtRegs() and for some examples even brought a slow down.

Patch by Vaidas Gasiunas <vaidas.gasiunas@sap.com>

Differential Revision: http://reviews.llvm.org/D6013

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228421 91177308-0d34-0410-b5e6-96231b3b80d8