[Bug tree-optimization/85697] At -Os nontrivial ctor does not use SSE to zero

[msharov at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85697

--- Comment #2 from Mike Sharov <msharov at users dot sourceforge.net> ---
I previously filed bug #49127 about the non-SSE version of the same xor/mov
optimization. Perhaps both could be addressed in the same manner with a more
general capability of zeroing with a register when doing so is shorter.