Add LiveInterval::find and use it for most LiveRange searching operations

instead of calling lower_bound or upper_bound directly.

This cleans up the search logic a bit because {lower,upper}_bound compare
LR->start by default, and it is usually simpler to search LR->end.

Funnelling all searches through one function also makes it possible to replace
the search algorithm with something faster than binary search.

llvm-svn: 114448