Fill bitregion_{start,end} in store_constructor (PR tree-optimization/78428)

	* expr.c (store_constructor_field): Add new arguments to the
	function.
	(store_constructor): Set up bitregion_end and add
	gcc_unreachable to fields that have either non-constant size
	or (and) offset.
	* gcc.dg/tree-ssa/pr78428.c: New test.

From-SVN: r243610