cmd/compile: ensure that loop condition is detected correctly

cmd/compile: ensure that loop condition is detected correctly

We need to make sure that the terminating comparison has the right
sense given the increment direction. If the increment is positive,
the terminating comparsion must be < or <=. If the increment is
negative, the terminating comparison must be > or >=.

Do a few cleanups,  like constant-folding entry==0, adding comments,
removing unused "exported" fields.

Fixes #26116

Change-Id: I14230ee8126054b750e2a1f2b18eb8f09873dbd5
Reviewed-on: https://go-review.googlesource.com/121940
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>