If we've already hit an Unimplemented, there may be important
SSA invariants that do not hold and which could cause
ssa.Compile to hang or spin.

While we're here, make detected dependency cycles stop execution.

Change-Id: Ic7d4eea659e1fe3f2c9b3e8a4eee5567494f46ad
Reviewed-on: https://go-review.googlesource.com/12310Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>