Fix depth field bug and resize() function in hierarchical barrier

This is a follow up to the hierarchy cleanup patch.
Added some clarifying comments to hierarchy_info.
Fixed a bug with the depth field not being updated cleanly during a resize.
Fixed resize to first check capacity as determined by maxLevels before actually doing the full resize.

Differential Revision: http://reviews.llvm.org/D12562

llvm-svn: 247333