target/sh4: Update DisasContextBase.insn_start

Match the extra inserts of INDEX_op_insn_start, fixing
the db->num_insns != 1 assert in translator_loop.

Fixes: dcd092a063 ("accel/tcg: Improve can_do_io management")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

target/sh4/translate.c

@@ -2189,6 +2189,7 @@ static void decode_gusa(DisasContext *ctx, CPUSH4State *env)
      */
     for (i = 1; i < max_insns; ++i) {
         tcg_gen_insn_start(pc + i * 2, ctx->envflags);
+        ctx->base.insn_start = tcg_last_op();
     }
 }
 #endif