[mips64] Change scratch register from kScratchReg to at in JumpIfSmi

This CL fixes tests failure caused by register allocation conflicts.

Change-Id: Ie5ba445d2e36047abf95644b613f49d3ed47315f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2963492Reviewed-by: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Auto-Submit: Liu yu <liuyu@loongson.cn>
Cr-Commit-Position: refs/heads/master@{#75154}

src/codegen/mips64/macro-assembler-mips64.h

@@ -813,8 +813,8 @@ class V8_EXPORT_PRIVATE TurboAssembler : public TurboAssemblerBase {
   void MSARoundD(MSARegister dst, MSARegister src, FPURoundingMode mode);
 
   // Jump the register contains a smi.
-  void JumpIfSmi(Register value, Label* smi_label,
-                 Register scratch = kScratchReg, BranchDelaySlot bd = PROTECT);
+  void JumpIfSmi(Register value, Label* smi_label, Register scratch = at,
+                 BranchDelaySlot bd = PROTECT);
 
   void JumpIfEqual(Register a, int32_t b, Label* dest) {
     li(kScratchReg, Operand(b));