[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