vect: Don't set excess bits in unform masks

vect: Don't set excess bits in unform masks

AVX ignores any excess bits in the mask (at least for vector sizes >=8), but
AMD GCN magically uses a larger vector than was intended (the smaller sizes are
"fake"), leading to wrong-code.

This patch fixes amdgcn execution failures in gcc.dg/vect/pr81740-1.c,
gfortran.dg/c-interop/contiguous-1.f90,
gfortran.dg/c-interop/ff-descriptor-7.f90, and others.

gcc/ChangeLog:

	* expr.cc (store_constructor): Add "and" operation to uniform mask
	generation.