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

[segher at gcc dot gnu.org]

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

--- Comment #8 from Segher Boessenkool <segher at gcc dot gnu.org> ---
It is not fixed in trunk, even.  A better testcase removes the __volatile__:
if this is properly optimised the whole asm disappears then, but in the case
of MYSIZE 3 it does not with the current GCC.