cmd/compile: prevent memmove in copy when dst == src

cmd/compile: prevent memmove in copy when dst == src

This causes a nominal increase in binary size.

name        old object-bytes  new object-bytes  delta
Template          399kB ± 0%        399kB ± 0%    ~     (all equal)
Unicode           207kB ± 0%        207kB ± 0%    ~     (all equal)
GoTypes          1.23MB ± 0%       1.23MB ± 0%    ~     (all equal)
Compiler         4.35MB ± 0%       4.35MB ± 0%  +0.01%  (p=0.008 n=5+5)
SSA              9.77MB ± 0%       9.77MB ± 0%  +0.00%  (p=0.008 n=5+5)
Flate             236kB ± 0%        236kB ± 0%  +0.04%  (p=0.008 n=5+5)
GoParser          298kB ± 0%        298kB ± 0%    ~     (all equal)
Reflect          1.03MB ± 0%       1.03MB ± 0%  +0.01%  (p=0.008 n=5+5)
Tar               333kB ± 0%        334kB ± 0%  +0.22%  (p=0.008 n=5+5)
XML               414kB ± 0%        414kB ± 0%  +0.02%  (p=0.008 n=5+5)
[Geo mean]        730kB             731kB       +0.03%

Change-Id: I381809fd9cfbfd6db44bd342b06285e62a3a21f1
Reviewed-on: https://go-review.googlesource.com/94596
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>