[llvm][CodeGen][SVE] Constrain prefetch intrinsic argument to immediate values.

Summary:
The argument that sets the prefetch type of a prefetch intrinsic must
be an immediate value.

Reviewers: andwar, sdesmalen, efriedma

Subscribers: tschuett, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75482