For stabs debug info it was used to suppress debug info, so set ir_visibility_private instead.

entity_is_externally_visible() checks for IR_LINKAGE_HIDDEN_USER, too.

This fixes backend/destam_flags2.c.

This fixes opt/fehler285.c.

This is the first half for properly supporting switch in PIC.
The entries of the table need to be adapted to PIC, too.

This fixes opt/fehler302.c.

This fixes opt/reassoc039.c.

This allows to optimize conversion like Iu -> Lu -> F.

Also remove the nonsensical parameter pn, which is the Proj num of the old flags producer, from the try_replace callback.

The Proj might exist already, so reuse it.
If it needs to be created, then do this once, not once per flags uers.

In this case the list of flags uers was erroneously reset.
But the new flags producer is not definitely available, so it might be necessary to rewire the users again.

This removes the special cases in amd64_has_addr_attr(), which corrected for the wrong op_mode.

Now the pos for fetching an out info is checked in all cases.

This fixes backend/amd64_sub2.c.