cselib: dump_cselib_* fixes

cselib: dump_cselib_* fixes

Rework dump_cselib_table to not crash when cselib_preserved_hash_table
is not allocated, and to remove the extraneous indirection from
dump_cselib_val that made it inconvenient to call from a debugger.

for  gcc/ChangeLog

	* cselib.cc (dump_cselib_val): Split out of and rename to...
	(dump_cselib_val_ptr): ... this.
	(dump_cselib_table): Adjust.  Skip cselib_preserved_hash_table
	when not allocated.