More .DEFERRED_INIT expansion rework

More .DEFERRED_INIT expansion rework

This avoids looking at the type size and instead uses the size
as passed to .DEFERRED_INIT to determine the size of the non-MEM
to be initialized.  It also arranges for possibly poly-int
inits to always use zero-initialization rather than not initializing
and when we need to pun puns the LHS instead of the constant value.

That correctly initializes the variable-size typed array in the
testcase for PR102285 and the SVE vector in PR102587 where for
the testcase I needed to add a SVE capable -march as to not
ICE later.

2021-10-05  Richard Biener  <rguenther@suse.de>

	PR middle-end/102587
	PR middle-end/102285
	* internal-fn.c (expand_DEFERRED_INIT): Fall back to
	zero-initialization as last resort, use the constant
	size as given by the DEFERRED_INIT argument to build
	the initializer.

	* gcc.target/aarch64/sve/pr102587-1.c: Add -march=armv8.3-a+sve.
	* gcc.target/aarch64/sve/pr102587-2.c: Likewise.