[Bug c++/93557] __builtin_convertvector doesn't mark input as used

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93557

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 47776
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47776&action=edit
gcc10-pr93557.patch

Untested fix.