Somewhat notably, this means long if statement chains are now parsed
recursively, rather than iteratively.  This shouldn't be a concern
though, as several other functions (e.g., gen, typecheck, walk)
already use recursion to process the parsed if statement Node trees.

Change-Id: Ic8c12ace9021c870d60c06f5db86a48c4ec57084
Reviewed-on: https://go-review.googlesource.com/19756Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>