Fix ICE. [PR103682]

Check is_gimple_assign before gimple_assign_rhs_code.

gcc/ChangeLog:

	PR target/103682
	* tree-ssa-ccp.c (optimize_atomic_bit_test_and): Check
	is_gimple_assign before gimple_assign_rhs_code.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/compile/pr103682.c: New test.