This patch builds upon http://reviews.llvm.org/D20178 and speeds up LiveDebugValues::transferDebugValue() by adding an index that maps each DebugVariable to its open VarLoc.
The transferDebugValue() function needs to close all open ranges for a given DebugVariable. Iterating over the set bits of OpenRanges is prohibitively slow in practice. I experimented with using the sorted map of VarLocs in the UniqueVector to iterate only over the range of VarLocs with a given DebugVariable, but the binary search turned out to be even more expensive than just iterating over the set bits in OpenRanges.
Instead, this patch exploits the fact that there can only be one open location for each DebugVariable and redundantly stores this location in a DenseMap.
bench1: The ASAN -O3 example I've been testing with.
bench2: The bitcode from PR26055 at -O0.
bench1: user 2m43.600s
bench2: user 1m0.948s
bench1: user 1m6.725s
bench2: user 0m21.452s