[Bug tree-optimization/60770] disappearing clobbers

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60770

--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
Created attachment 33024
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33024&action=edit
replace clobber with default def

This passes bootstrap+testsuite with all default languages, but it breaks ada,
I get plenty of warnings (as if it was reading objects after clobbering them)
and 2 errors about "Unable to coalesce ssa_names [...] which are marked as MUST
COALESCE."

I am probably doing something wrong in the patch, but ada seems to be using
clobbers differently than others since it is the only one that notices (well, I
didn't have a chance to check go since ada broke early).

It would also be nice to save the info somewhere that this default def actually
comes from a clobber, so the uninit pass can give a different warning message.
Maybe there is a bit available in the gimple_nop...