Replace calls to memmove with known (constant) size, with OpMove.
Do it only if it is safe from aliasing point of view.
Helps with code like this:

append(buf,"const str"...)

In strconv this provides nice benefit:
Quote-6                                   731ns ± 2%   647ns ± 3%  -11.41%  (p=0.000 n=10+10)
QuoteRune-6                               117ns ± 5%   111ns ± 1%   -4.54%  (p=0.000 n=10+10)
AppendQuote-6                             475ns ± 0%   396ns ± 0%  -16.59%  (p=0.000 n=9+10)
AppendQuoteRune-6                        32.0ns ± 0%  27.4ns ± 0%  -14.41%  (p=0.000 n=8+9)

Change-Id: I7704f5c51b46aed2d8f033de74c75140fc35036c
Reviewed-on: https://go-review.googlesource.com/54394
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>