[analyzer] Use collectSubRegionKeys to make removeDeadBindings faster.

Previously, whenever we had a LazyCompoundVal, we crawled through the
entire store snapshot looking for bindings within the LCV's region. Now, we
just ask for the subregion bindings of the lazy region and only visit those.

This is an optimization (so no test case), but it may allow us to clean up
more dead bindings than we were previously.

llvm-svn: 175230