Don't require that the fallthrough instruction has an encoding.

A fallthrough jump is actually represented as 0 bytes, so no encoding is
needed.

Also allow for unencoded instructions in the generated emit_inst
implementations. The verifier has stricter rules for when this is
allowed.

cranelift/filetests/wasm/control.cton

@@ -32,3 +32,14 @@ ebb1(v2: i32):
 ebb2:
     jump ebb1(v0)
 }
+
+function %br_if_fallthrough(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 1
+    brz v0, ebb1(v1)
+    ; This jump gets converted to a fallthrough.
+    jump ebb1(v0)
+
+ebb1(v2: i32):
+    return v2
+}