[Bug inline-asm/63900] memory constrains needlessly doing memory clobber

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63900

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #3)
> Reopened; this is a perfectly valid missed optimisation.
> 
> Nothing here does a "full memory clobber"; instead, CSE simply doesn't
> know how to handle a three-byte datum.

That is because it is BLKmode.  This is most likely going to be a won't fix
really because it is too hard to implement for small benefit.

Also in this case using memset would be better and optimize easier.