If the values being compared have different concrete types,
then they're clearly unequal without needing to invoke the
actual interface compare routine.  This speeds tests for
specific values, like if err == io.EOF, by about 3x.

benchmark                  old ns/op    new ns/op    delta
BenchmarkIfaceCmp100             843          287  -65.95%
BenchmarkIfaceCmpNil100          184          182   -1.09%

Fixes #2591.

R=ken2
CC=golang-dev
https://golang.org/cl/5651073