We already require expressions to have already been typechecked before
reaching walk. Moreover, all untyped expressions should have been
converted to their default type by walk.

However, in practice, we've been somewhat sloppy and inconsistent
about ensuring this. In particular, a lot of AST rewrites ended up
leaving untyped bool expressions scattered around. These likely aren't
harmful in practice, but it seems worth cleaning up.

The two most common cases addressed by this CL are:

1) When generating OIF and OFOR nodes, we would often typecheck the
conditional expression, but not apply defaultlit to force it to the
expression's default type.

2) When rewriting string comparisons into more fundamental primitives,
we were simply overwriting r.Type with the desired type, which didn't
propagate the type to nested subexpressions. These are fixed by
utilizing finishcompare, which correctly handles this (and is already
used by other comparison lowering rewrites).

Lastly, walkexpr is extended to assert that it's not called on untyped
expressions.

Fixes #23834.

Change-Id: Icbd29648a293555e4015d3b06a95a24ccbd3f790
Reviewed-on: https://go-review.googlesource.com/98337Reviewed-by: Robert Griesemer <gri@golang.org>