[AArch64][GlobalISel] Select @llvm.aarch64.ldaxr.* intrinsics

Add a GISelPredicateCode to ldaxr_*. This allows us to import the patterns for
@llvm.aarch64.ldaxr.*, and thus select them.

Add `isLoadStoreOfNumBytes` for the GISelPredicateCode, since each of these
intrinsics involves the same check.

Add select-ldaxr-intrin.mir, and update arm64-ldxr-stxr.ll.

Differential Revision: https://reviews.llvm.org/D66897

llvm-svn: 370377