The existing code for encoding 'for' loops in exported, inlineable
functions incorrectly assumed that the 'Right' field points to an
'expression' node. Adjusted the code to be able to handle any kind
of node. Made matching changes for the binary and indexed exporter.

This only shows up together with other pending compiler changes that
enable exporting of such functions which contain for loops.

No tests yet because we can't test this w/o those pending compiler
changes. Once those changes are in, this code will be tested implicitly.
However, the changes were tested manually together with the patches
described in the issue.

Fixes #25222.

Change-Id: I54babb87e5d665d2c1ef6116c1de1b8c50b1138e
Reviewed-on: https://go-review.googlesource.com/119595Reviewed-by: David Chase <drchase@google.com>