plugins: Use emit_before_op for PLUGIN_GEN_AFTER_INSN

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>