go.tools/oracle: callgraph: fix crash caused by sparse node numbering.
go.tools/oracle: callgraph: fix crash caused by sparse node numbering.

Revision 0ea4058a1ca3 caused the node numbering to become
sparse, violating a precondition of
(*callgraphResult).toSerial.  Now we renumber the callgraph
nodes always, not just the qpos != nil case.

Fixes issue 8171

LGTM=crawshaw
R=crawshaw
CC=golang-codereviews
https://codereview.appspot.com/103240044

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old