Stop representing scalars as explicit nodes in the graph. Now the only

nodes in the graph are memory objects, which is very nice.  This also greatly
reduces the size and memory footprint for DSGraphs.  For example, the local
DSGraph for llu went from 65 to 13 nodes with this change.  As a side bonus,
dot seems to lay out the graphs slightly better too.  :)

llvm-svn: 4488