Optimise hash_table::empty

Optimise hash_table::empty

Calling redirect_edge_var_map_empty after each pass was slowing things
down because hash_table::empty () cleared all slots even if the hash
table was already empty.

Tested on x86_64-linux-gnu, where it gives a 1% compile time improvement
for fold-const.ii at -O and -O2.

gcc/
	* hash-table.h (hash_table::empty): Turn into an inline wrapper
	that checks whether the table is already empty.  Rename the
	original implementation to...
	(hash_table::empty_slot): ...this new private function.

From-SVN: r232467