[mips64][codegen] Fix the optimization for long branch

If a long branch doesn't use delay slot, then when optimizing it
to a regular branch, the instruction in delay slot should be set
to nop.

Change-Id: Id3015bc0c562725258705a8bc6647c4011d96c2a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2524416Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Cr-Commit-Position: refs/heads/master@{#71039}

src/codegen/mips64/assembler-mips64.cc

@@ -878,8 +878,10 @@ void Assembler::target_at_put(int pos, int target_pos, bool is_internal) {
       Instr instr_branch_delay;
 
       if (IsJump(instr_j)) {
-        instr_branch_delay = instr_at(pos + 6 * kInstrSize);
+        // Case when branch delay slot is protected.
+        instr_branch_delay = nopInstr;
       } else {
+        // Case when branch delay slot is used.
         instr_branch_delay = instr_at(pos + 7 * kInstrSize);
       }
       instr_at_put(pos, instr_b);