(Here, quoted strings are the official AMD names.)

The amd64 "movsxd" instruction, when invoked
with a 64-bit REX prefix, moves and sign extends
a 32-bit value from register or memory into a
64-bit register.  6.out.h spells this MOVLQSX.

6.out.h also includes MOVLQZX, the zero extending
version, which it implements as "movsxd" without
the REX prefix.  Without the REX prefix it's only sign
extending 32 bits to 32 bits (i.e., not doing anything
to the bits) and then storing in a 32-bit register.
Any write to a 32-bit register zeros the top half of the
corresponding 64-bit register, giving the advertised effect.
This particular implementation of the functionality is
non-standard, because an ordinary 32-bit "mov" would
do the same thing.

Because it is non-standard, it is often mishandled or
not handled by binary translation tools like valgrind.
Switching to the standard "mov" makes the binaries
work better with those tools.

It's probably useful in 6c and 6g to have an explicit
instruction, though, so that the intent of the size
change is clear.  Thus we leave the concept of MOVLQZX
and just implement it by the standard "mov" instead of
the non-standard 32-bit "movsxd".

Fixes #896.

R=ken2
CC=golang-dev
https://golang.org/cl/1733046