Simplify fold_single_bit_test slightly

Simplify fold_single_bit_test slightly

Now the only use of fold_single_bit_test is in do_store_flag,
we can change it such that to pass the inner arg and bitnum
instead of building a tree. There is no code generation changes
due to this change, only a decrease in GC memory that is produced
during expansion.

gcc/ChangeLog:

	* expr.cc (fold_single_bit_test): Take inner and bitnum
	instead of arg0 and arg1. Update the code.
	(do_store_flag): Don't create a tree when calling
	fold_single_bit_test instead just call it with the bitnum
	and the inner tree.