Move fold_single_bit_test to expr.cc from fold-const.cc

Move fold_single_bit_test to expr.cc from fold-const.cc

This is part 1 of N patch set that will change the expansion
of `(A & C) != 0` from using trees to directly expanding so later
on we can do some cost analysis.

Since the only user of fold_single_bit_test is now
expand, move it to there.

gcc/ChangeLog:

	* fold-const.cc (fold_single_bit_test_into_sign_test): Move to
	expr.cc.
	(fold_single_bit_test): Likewise.
	* expr.cc (fold_single_bit_test_into_sign_test): Move from fold-const.cc
	(fold_single_bit_test): Likewise and make static.
	* fold-const.h (fold_single_bit_test): Remove declaration.