mapaccess and mapdelete functions are already optimized to prefer direct
key comparison instead of tophash checks when key comparison is cheap.

Extended version of golang.org/cl/55235.

AMD64:
name                old time/op    new time/op    delta
MapPopulate/1         42.5ns ± 2%    40.3ns ± 2%  -5.37%  (p=0.000 n=9+10)
MapPopulate/10         558ns ± 1%     556ns ± 1%    ~     (p=0.157 n=10+10)
MapPopulate/100       7.75µs ± 1%    7.66µs ± 2%  -1.19%  (p=0.001 n=10+10)
MapPopulate/1000      92.6µs ± 1%    92.0µs ± 1%  -0.61%  (p=0.016 n=10+8)
MapPopulate/10000      817µs ± 1%     814µs ± 1%    ~     (p=0.247 n=10+10)
MapPopulate/100000    8.02ms ± 1%    7.90ms ± 2%  -1.47%  (p=0.007 n=10+10)

Change-Id: If0eca9931379cbbd37eb753e9bcd2888d8272153
Reviewed-on: https://go-review.googlesource.com/62050
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>