Make sure we don't do O(n^2) work to eliminate a chain
of n copies.

benchmark                     old ns/op       new ns/op     delta
BenchmarkCopyElim1-8          1418            1406          -0.85%
BenchmarkCopyElim10-8         5289            5162          -2.40%
BenchmarkCopyElim100-8        52618           41684         -20.78%
BenchmarkCopyElim1000-8       2473878         424339        -82.85%
BenchmarkCopyElim10000-8      269373954       6367971       -97.64%
BenchmarkCopyElim100000-8     31272781165     104357244     -99.67%

Change-Id: I680f906f70f2ee1a8615cb1046bc510c77d59284
Reviewed-on: https://go-review.googlesource.com/22535Reviewed-by: Alexandru Moșoi <alexandru@mosoi.ro>