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/shared/instructions.rs

@@ -580,6 +580,7 @@ pub(crate) fn define(
             .ints(Interval::All)
             .floats(Interval::All)
             .simd_lanes(Interval::All)
+            .refs(Interval::All)
             .build(),
     );