[LAA] Rename IsRTNeeded to IsRTCheckAnalysisNeeded

The original name was too close to NeedRTCheck which is what the actual
memcheck analysis returns.  This flag, as the new name suggests, is only
used to whether to initiate that analysis.

Also a comment is added to answer one question I had about this code for
a long time.  Namely, how does this flag differ from
isDependencyCheckNeeded since they are seemingly set at the same time.

llvm-svn: 241784