PR tree-optimization/16437
	* c-common.c (shorten_compare): Don't mark result of conversion to
	narrower signed type as overflowing.
	* fold-const.c (decode_field_reference): Determine whether
	signedness comes from outer type using precision rather than size.

testsuite:
	* gcc.c-torture/execute/bitfld-4.c: New test.

From-SVN: r84498