Add Intel encodings for sextend and uextend.

filetests/wasm/conversions.cton

@@ -9,3 +9,15 @@ ebb0(v0: i64):
     v1 = ireduce.i32 v0
     return v1
 }
+
+function %i64_extend_s_i32(i32) -> i64 {
+ebb0(v0: i32):
+    v1 = sextend.i64 v0
+    return v1
+}
+
+function %i64_extend_u_i32(i32) -> i64 {
+ebb0(v0: i32):
+    v1 = uextend.i64 v0
+    return v1
+}