aarch64, testsuite: Fix up auto-init-padding tests

aarch64, testsuite: Fix up auto-init-padding tests

The tests currently depend on memcpy lowering forming stps at -O0,
but we no longer want to form stps during memcpy lowering, but instead
in the upcoming load/store pair fusion pass.

This patch therefore tweaks affected tests to enable optimizations
(-O1), and adjusts the tests to avoid parts of the structures being
optimized away where necessary.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/auto-init-padding-1.c: Add -O to options,
	adjust test to work with optimizations enabled.
	* gcc.target/aarch64/auto-init-padding-2.c: Add -O to options.
	* gcc.target/aarch64/auto-init-padding-3.c: Add -O to options,
	adjust test to work with optimizations enabled.
	* gcc.target/aarch64/auto-init-padding-4.c: Likewise.
	* gcc.target/aarch64/auto-init-padding-9.c: Likewise.