Fix disambiguation against .MASK_STORE

Fix disambiguation against .MASK_STORE

Alias analysis was treating .MASK_STORE as storing a full vector
which means we disambiguate against decls of smaller than vector size.
That's of course wrong and a similar issue was fixed for DSE already.
The following makes sure we set the size of the access to unknown
and only constrain max_size.

This fixes runtime execution FAILs of gfortran.dg/matmul_2.f90,
gfortran.dg/matmul_6.f90 and gfortran.dg/pr91577.f90 when using
AVX512 with full masked loop vectorization on Zen4.

	* tree-ssa-alias.cc (call_may_clobber_ref_p_1): For
	.MASK_STORE and friend set the size of the access to
	unknown.