[SveEmitter] Add builtins for contiguous prefetches

This patch also adds the enum `sv_prfop` for the prefetch operation specifier
and checks to ensure the passed enum values are valid.

Reviewers: SjoerdMeijer, efriedma, ctetreau

Reviewed By: efriedma

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78674