tree-optimization/120817 - bogus DSE of .MASK_STORE

tree-optimization/120817 - bogus DSE of .MASK_STORE

DSE used ao_ref_init_from_ptr_and_size for .MASK_STORE but
alias-analysis will use the specified size to disambiguate
against smaller objects.  For .MASK_STORE we instead have to
make the access size unspecified but we can still constrain
the access extent based on the maximum size possible.

	PR tree-optimization/120817
	* tree-ssa-dse.cc (initialize_ao_ref_for_dse): Use
	ao_ref_init_from_ptr_and_range with unknown size for
	.MASK_STORE and .MASK_LEN_STORE.

	* gcc.dg/vect/pr120817.c: New testcase.