The Node type ODOT and its variants all represent a selector, with a
simple name to the right of the dot.  Before this change this was
represented by using an ONAME Node in the Right field.  This ONAME node
served no useful purpose.  This CL changes these Node types to store the
symbol in the Sym field instead, thus not requiring allocating a Node
for each selector.

When compiling x/tools/go/types this CL eliminates nearly 5000 calls to
newname and reduces the total number of Nodes allocated by about 6.6%.
It seems to cut compilation time by 1 to 2 percent.

Getting this right was somewhat subtle, and I added two dubious changes
to produce the exact same output as before.  One is to ishairy in
inl.go: the ONAME node increased the cost of ODOT and friends by 1, and
I retained that, although really ODOT is not more expensive than any
other node.  The other is to varexpr in walk.go: because the ONAME in
the Right field of an ODOT has no class, varexpr would always return
false for an ODOT, although in fact for some ODOT's it seemingly ought
to return true; I added an && false for now.  I will send separate CLs,
that will break toolstash -cmp, to clean these up.

This CL passes toolstash -cmp.

Change-Id: I4af8a10cc59078c436130ce472f25abc3a9b2f80
Reviewed-on: https://go-review.googlesource.com/20890
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>