AArch64: port load and store operations to ISLE. (#4785)

This retains `lower_amode` in the handwritten code (@akirilov-arm
reports that there is an upcoming patch to port this), but tweaks it
slightly to take a `Value` rather than an `Inst`.

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -5,7 +5,7 @@ use crate::ir::types;
 use crate::ir::types::*;
 use crate::ir::MemFlags;
 use crate::ir::Opcode;
-use crate::ir::{ExternalName, LibCall, Signature};
+use crate::ir::{dynamic_to_fixed, ExternalName, LibCall, Signature};
 use crate::isa;
 use crate::isa::aarch64::{inst::EmitState, inst::*, settings as aarch64_settings};
 use crate::isa::unwind::UnwindInst;