reflect: optimize DeepEqual() for maps

reflect: optimize DeepEqual() for maps

benchmark                     old ns/op     new ns/op     delta
BenchmarkMapsDeepEqual-10     235           200           -15.05%

benchmark                     old allocs     new allocs     delta
BenchmarkMapsDeepEqual-10     7              6              -14.29%

benchmark                     old bytes     new bytes     delta
BenchmarkMapsDeepEqual-10     96            48            -50.00%

Change-Id: Ifa625ad25524cc9ee438711917606626b33a9597
Reviewed-on: https://go-review.googlesource.com/c/go/+/512576
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>