sha1-mb-x86_64.pl: add commentary.

my $j=$i+1;

my $k=$i+2;

# Loads are performed 2+3/4 iterations in advance. 3/4 means that out

# of 4 words you would expect to be loaded per given iteration one is

# spilled to next iteration. In other words indices in four input

# streams are distributed as following:

#

# $i==0:	0,0,0,0,1,1,1,1,2,2,2,

# $i==1:	2,3,3,3,

# $i==2:	3,4,4,4,

# ...

# $i==13:	14,15,15,15,

# $i==14:	15

# 

# Then at $i==15 Xupdate is applied one iteration in advance...

$code.=<<___ if ($i==0);

	movd		(@ptr[0]),@Xi[0]

	 lea		`16*4`(@ptr[0]),@ptr[0]