Add a null encoding for ireduce.i32.i64.

This conversion doesn't require any code, we're just looking at the bits
differently.

cranelift/filetests/wasm/conversions.cton

@@ -0,0 +1,11 @@
+; Test code generation for WebAssembly type conversion operators.
+test compile
+
+set is_64bit=1
+isa intel haswell
+
+function %i32_wrap_i64(i64) -> i32 {
+ebb0(v0: i64):
+    v1 = ireduce.i32 v0
+    return v1
+}