cmd/compile: correct alias cycle detection

The original fix (https://go-review.googlesource.com/c/go/+/35831)
for this issue was incorrect as it reported cycles in cases where
it shouldn't.

Instead, use a different approach: A type cycle containing aliases
is only a cycle if there are no type definitions. As soon as there
is a type definition, alias expansion terminates and there is no
cycle.

Approach: Split sprint_depchain into two non-recursive and more
easily understandable functions (cycleFor and cycleTrace),
and use those instead for cycle reporting. Analyze the cycle
returned by cycleFor before issueing an alias cycle error.

Also: Removed original fix (main.go) which introduced a separate
crash (#23823).

Fixes #18640.
Fixes #23823.
Fixes #24939.

Change-Id: Ic3707a9dec40a71dc928a3e49b4868c5fac3d3b7
Reviewed-on: https://go-review.googlesource.com/118078


Reviewed-by: Matthew Dempsky <mdempsky@google.com>