Add -fbit-tests option.

gcc/ChangeLog:

	* common.opt: Add new -fbit-tests option.
	* doc/invoke.texi: Document the option.
	* tree-switch-conversion.c (bit_test_cluster::find_bit_tests):
	Use the option.
	* tree-switch-conversion.h (is_enabled): New function.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/switch-4.c: New test.