[llvm][CodeGen] Fix issue for SVE gather prefetch.

Summary:
This change is fixing an issue where the dagcombine incorrectly used an addressing mode with scaled offsets (indices), instead of unscaled offsets.
Those addressing modes do not exist for `prfh` , `prfw` and `prfd`, hence we can reuse `prfb` because that has unscaled offsets, and because the pseudo-code in the XML spec suggests that the element size is not used for the amount of data that is prefetched by the instruction.

FWIW, GCC also emits a `prfb` for these cases.

Reviewers: sdesmalen, andwar, rengolin

Reviewed By: sdesmalen

Subscribers: tschuett, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78069