PR94600: fix volatile access to the whole of a compound object.

PR94600: fix volatile access to the whole of a compound object.

The store to the whole of each volatile object was picked apart
like there had been an individual assignment to each of the
fields.  Reads were added as part of that; see PR for details.
The reads from volatile memory were a clear bug; individual
stores questionable.  A separate patch clarifies the docs.

gcc:

2020-07-09  Richard Biener  <rguenther@suse.de>

	PR middle-end/94600
	* expr.c (expand_constructor): Make a temporary also if we're
	storing to volatile memory.

gcc/testsuite:

2020-07-09  Hans-Peter Nilsson  <hp@axis.com>

	PR middle-end/94600
	* gcc.dg/pr94600-1.c, gcc.dg/pr94600-2.c, gcc.dg/pr94600-3.c,
	gcc.dg/pr94600-4.c, gcc.dg/pr94600-5.c, gcc.dg/pr94600-6.c,
	gcc.dg/pr94600-7.c, gcc.dg/pr94600-8.c: New tests.