[expr.c] PR middle-end/71700: zero-extend sub-word value when widening constructor...

[expr.c] PR middle-end/71700: zero-extend sub-word value when widening constructor element

	Backport from mainline
	2016-07-12  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	PR middle-end/71700
	* expr.c (store_constructor): Mask sign-extended bits when widening
	sub-word constructor element at the start of a word.

	* gcc.c-torture/execute/pr71700.c: New test.

From-SVN: r239754