re PR tree-optimization/52979 (likely wrong code bug w/packed bitfields)

re PR tree-optimization/52979 (likely wrong code bug w/packed bitfields)

	PR middle-end/52979
	* stor-layout.c (get_best_mode): Don't return mode with bitsize
	larger than maxbits.  Don't compute maxbits modulo align.
	Also check that unit bytes long store at bitpos / unit * unit
	doesn't affect bits beyond bitregion_end.
	* expmed.c (store_bit_field_1): Avoid trying insv if OP_MODE MEM
	would not fit into bitregion_start ... bitregion_end + 1 bit
	region.
	(store_split_bit_field): Decrease unit close to end of bitregion_end
	if access is restricted in order to avoid mutual recursion.

	* gcc.c-torture/compile/pr52979-1.c: New test.
	* gcc.c-torture/execute/pr52979-1.c: New test.
	* gcc.c-torture/execute/pr52979-2.c: New test.

From-SVN: r186819