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/ir/mod.rs

@@ -35,7 +35,7 @@ pub use crate::ir::builder::{
 };
 pub use crate::ir::constant::{ConstantData, ConstantPool};
 pub use crate::ir::dfg::{DataFlowGraph, ValueDef};
-pub use crate::ir::dynamic_type::{DynamicTypeData, DynamicTypes};
+pub use crate::ir::dynamic_type::{dynamic_to_fixed, DynamicTypeData, DynamicTypes};
 pub use crate::ir::entities::{
     Block, Constant, DynamicStackSlot, DynamicType, FuncRef, GlobalValue, Heap, Immediate, Inst,
     JumpTable, SigRef, StackSlot, Table, UserExternalNameRef, Value,