tree-optimization/106904 - bogus -Wstringopt-overflow with vectors

The following avoids CSE of &ps->wp to &ps->wp.hwnd confusing
-Wstringopt-overflow by making sure to produce addresses to the
biggest container from vectorization.  For this I introduce
strip_zero_offset_components which turns &ps->wp.hwnd into
&(*ps) and use that to base the vector data references on.
That will also work for addresses with variable components,
alternatively emitting pointer arithmetic via calling
get_inner_reference and gimplifying that would be possible
but likely more intrusive.

This is by no means a complete fix for all of those issues
(avoiding ADDR_EXPRs in favor of pointer arithmetic might be).
Other passes will have similar issues.

In theory that might now cause false negatives.

	PR tree-optimization/106904
	* tree.h (strip_zero_offset_components): Declare.
	* tree.cc (strip_zero_offset_components): Define.
	* tree-vect-data-refs.cc (vect_create_addr_base_for_vector_ref):
	Strip zero offset components before building the address.

	* gcc.dg/Wstringop-overflow-pr106904.c: New testcase.

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