lto/106334 - relax assert during WPA tree merging

lto/106334 - relax assert during WPA tree merging

The dwarf2out map of tree to symbol + offset is populated too early
when streaming in trees so that when WPA tree merging decides to
recycle them the mapping prevails and if we are unlucky the same
address is used for another tree with a symbol + offset DIE to
record.  The following mitigates the resulting ICE by relaxing the
assert, allowing re-use of a slot during WPA.  Delaying the register
would be better but it's already somewhat hairy and uglifying this
further doesn't look too important right now.

	PR lto/106334
	* dwarf2out.cc (dwarf2out_register_external_die): Allow
	map entry re-use during WPA.