Codegen: Allow encoding of (r32|r64).(load|store)

Accessing Wasm reference globals that are reference types will
want to use the plain load/store instructions. This commit adds
encodings for these instructions to match loading a i32/i64.
Producers of IR are required to insert the appropriate barriers
around the loads/stores.

cranelift/codegen/meta/src/cdsl/cpu_modes.rs

@@ -37,6 +37,12 @@ impl CpuMode {
         assert!(self.default_legalize.is_none());
         self.default_legalize = Some(group.id);
     }
+    pub fn legalize_value_type(&mut self, lane_type: impl Into<ValueType>, group: &TransformGroup) {
+        assert!(self
+            .typed_legalize
+            .insert(lane_type.into(), group.id)
+            .is_none());
+    }
     pub fn legalize_type(&mut self, lane_type: impl Into<LaneType>, group: &TransformGroup) {
         assert!(self
             .typed_legalize