plugins: Use emit_before_op for PLUGIN_GEN_AFTER_INSN

Introduce a new plugin_cb op and migrate one operation.
By using emit_before_op, we do not need to emit opcodes
early and modify them later -- we can simply emit the
final set of opcodes once.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/tcg-op.c

@@ -312,6 +312,11 @@ void tcg_gen_mb(TCGBar mb_type)
     }
 }
 
+void tcg_gen_plugin_cb(unsigned from)
+{
+    tcg_gen_op1(INDEX_op_plugin_cb, from);
+}
+
 void tcg_gen_plugin_cb_start(unsigned from, unsigned type, unsigned wr)
 {
     tcg_gen_op3(INDEX_op_plugin_cb_start, from, type, wr);