The tree vectorizer sometimes creates DEFs and removes them before the
end of the pass.  By the time of the removal, the SSA name is still
marked as needing updates, so we skip the code that might introduce a
debug temp before the DEF.

By the time we actually release the DEF, calling that code again, the
DEF is no longer in the stmt seq, so we can't tell where to insert the
debug temp.

We could just drop the debug info on the floor, but if the value is
unchanging, we can still propagate it to debug stmts that use that SSA
name.  This is what this patch does, also avoiding the ICE we got trying
to figure out where to insert the debug temp.

Regstrapped on x86_64-linux-gnu.  Ok to install?