target/90622: __builtin_avr_insert bits: Use BLD/BST for one bit in place.

target/90622: __builtin_avr_insert bits: Use BLD/BST for one bit in place.

If just one bit is inserted in the same position like with:
    __builtin_avr_insert_bits (0xFFFFF2FF, src, dst);
a BLD/BST sequence is better than XOR/AND/XOR.  Thus, don't fold that
case to the latter sequence.

gcc/
	PR target/90622
	* config/avr/avr.cc (avr_fold_builtin) [AVR_BUILTIN_INSERT_BITS]:
	Don't fold to XOR / AND / XOR if just one bit is copied to the
	same position.