[PATCH] Fix/Redo computeMainRangeFromSubranges()

[Matthias Braun]

Abandoning this revision for now:

- In the current pipeline where live intervals are calculated immediately after PhiElimination and TwoAddress calculation the structures are so simple that the current code is good enough. So no need for this yet.
- When doing things differently like recomputing whole live ranges in the register coalescer instead of updating them, a patch like this is necessary, but it turned out this is not enough. Without having detailed information about undef uses at the subregister level we still run into problems. So this approach can only work in combination with a scheme to mark partially undef uses.


http://reviews.llvm.org/D7887

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