[AArch64][SVE] Implement integer compare intrinsics

Summary:
Adds intrinsics for the following:

    * cmphs, cmphi
    * cmpge, cmpgt
    * cmpeq, cmpne
    * cmplt, cmple
    * cmplo, cmpls

Includes a minor change to `TLI.getMemValueType` that fixes a crash due to the
scalable flag being dropped.

Reviewers: sdesmalen, efriedma, rengolin, rovka, dancgr, huntergr

Reviewed By: efriedma

Subscribers: tschuett, kristof.beyls, hiraditya, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70889