memmove used to use 2 2-byte load/store pairs to move 4 bytes.
When the result is loaded with a single 4-byte load, it caused
a store to load fowarding stall.  To avoid the stall,
special case memmove to use 4 byte ops for the 4 byte copy case.

We already have a special case for 8-byte copies.
386 already specializes 4-byte copies.
I'll do 2-byte copies also, but not for 1.8.

benchmark                 old ns/op     new ns/op     delta
BenchmarkIssue18740-8     7567          4799          -36.58%

3-byte copies get a bit slower.  Other copies are unchanged.
name         old time/op   new time/op   delta
Memmove/3-8   4.76ns ± 5%   5.26ns ± 3%  +10.50%  (p=0.000 n=10+10)

Fixes #18740

Change-Id: Iec82cbac0ecfee80fa3c8fc83828f9a1819c3c74
Reviewed-on: https://go-review.googlesource.com/35567
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>