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/inst/args.rs

@@ -773,18 +773,3 @@ impl VectorSize {
         }
     }
 }
-
-pub(crate) fn dynamic_to_fixed(ty: Type) -> Type {
-    match ty {
-        I8X8XN => I8X8,
-        I8X16XN => I8X16,
-        I16X4XN => I16X4,
-        I16X8XN => I16X8,
-        I32X2XN => I32X2,
-        I32X4XN => I32X4,
-        I64X2XN => I64X2,
-        F32X4XN => F32X4,
-        F64X2XN => F64X2,
-        _ => unreachable!("unhandled type: {}", ty),
-    }
-}