The existing algorithm assumed that the length of a cycle was simply
the number of elements in the cycle slice starting at the start object.
However, we use a special "indir" indirection object to indicate
pointer and other indirections that break the inline placement of
types in composite types. These indirection objects don't exist as
true named type objects, so don't count them anymore.

This removes an unnecessary cycle error in one of the existing tests
(testdata/issues.src:100).

Also:
- added more tracing support (only active if tracing is enabled)
- better documentation in parts
- r/check.typ/check.typExpr/ in a few of places where we don't
  need to record a type indirection

Found while investigating #26124.

Change-Id: I45341743225d979a72af3fbecfa05012b32fab67
Reviewed-on: https://go-review.googlesource.com/121755
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>