[IR][VP] Remove IntrArgMemOnly from vp.gather/scatter.

IntrArgMemOnly is only valid for intrinsics that use a scalar
pointer argument. These intrinsics use a vector of pointer.

Alias analysis will try to find a scalar pointer argument and
will return incorrect alias results when it doesn't find one.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D133898