Fix disambiguation against .MASK_LOAD

Alias analysis was treating .MASK_LOAD as storing a full vector
which means we disambiguate against decls of smaller than vector size.
This complements the previous patch handling .MASK_STORE and fixes
runtime execution FAILs of gfortran.dg/matmul_3.f90 and
gfortran.dg/inline_sum_2.f90 when using AVX512 with full masked loop
vectorization on Zen4.

	* tree-ssa-alias.cc (ref_maybe_used_by_call_p_1): For
	.MASK_LOAD and friends set the size of the access to unknown.