Bug fix to merging of data flow values (merge incorrectly made values

too "conservative").

Several revisions to UninitializedValues checker after testing.  We
now appear to be working correctly (probably some bugs still, but main
functionality appears to be there).  Implemented careful emitting of
warnings so that we wouldn't get a cascade of warnings for simply not
defining a single variable and using it everywhere.  This way the
warnings point closer to the root cause rather than "symptoms" from
using values derived from uninitialized variables.

llvm-svn: 42067