Mips[64] Use kScratchReg instead of at register

At is used in Macro Assembler, so we need other registers to hold temporary values.

Change-Id: Iffeddba7b3319666a605eea62ecc3cd01b065ad7
Reviewed-on: https://chromium-review.googlesource.com/1013978Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Sreten Kovacevic <sreten.kovacevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#52623}

src/compiler/mips/code-generator-mips.cc

@@ -3373,8 +3373,8 @@ void CodeGenerator::AssembleMove(InstructionOperand* source,
         if (bit_cast<int32_t>(src.ToFloat32()) == 0) {
           __ sw(zero_reg, dst);
         } else {
-          __ li(at, Operand(bit_cast<int32_t>(src.ToFloat32())));
-          __ sw(at, dst);
+          __ li(kScratchReg, Operand(bit_cast<int32_t>(src.ToFloat32())));
+          __ sw(kScratchReg, dst);
         }
       } else {
         DCHECK(destination->IsFPRegister());

src/compiler/mips64/code-generator-mips64.cc

@@ -3627,8 +3627,8 @@ void CodeGenerator::AssembleMove(InstructionOperand* source,
         if (bit_cast<int32_t>(src.ToFloat32()) == 0) {
           __ Sw(zero_reg, dst);
         } else {
-          __ li(at, Operand(bit_cast<int32_t>(src.ToFloat32())));
-          __ Sw(at, dst);
+          __ li(kScratchReg, Operand(bit_cast<int32_t>(src.ToFloat32())));
+          __ Sw(kScratchReg, dst);
         }
       } else {
         DCHECK(destination->IsFPRegister());