go/callgraph: fix asymptote trap in DeleteSyntheticNodes.
go/callgraph: fix asymptote trap in DeleteSyntheticNodes.

The previous implementation would cause the graph to contain
many duplicate edges resulting in very large cross products,
so that for some inputs (and random map iteration orders) the
running time of DeleteSyntheticNodes was many minutes---more
than the pointer analysis!

Duplicate edges can arise from an interface call that
dispatches to several different wrapper functions each
wrapping the same declared method.

For example, in the callgraph for go/types, a call to
Object.Pos() dispatches to the synthetic functions (*Type).Pos
and (*Var).Pos, each of which wrap (*object).Pos().  After
DeleteSyntheticNodes, Object.Pos() appeared to call
(*object).Pos() twice.

This change builds the set of all edges and avoids adding
edges already in the set.

Also, document findings.

LGTM=crawshaw
R=crawshaw
CC=golang-codereviews
https://codereview.appspot.com/96100043

webfd/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,8limbs/gotools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,d0f/go-zh.tools