MIPS: Improvement of “MIPS: Fixed crashes exposed though fuzzing.” (r17886).

src/mips/macro-assembler-mips.cc

Index: src/mips/macro-assembler-mips.cc
diff --git a/src/mips/macro-assembler-mips.cc b/src/mips/macro-assembler-mips.cc
index 930afcb72abe8787d51955d2ca41235c27bd74ea..1ad6a38a85a837b9cb883b45d6d9cbaa23f5930f 100644
--- a/src/mips/macro-assembler-mips.cc
+++ b/src/mips/macro-assembler-mips.cc
@@ -5082,8 +5082,8 @@ void MacroAssembler::EmitSeqStringSetCharCheck(Register string,
   lw(at, FieldMemOperand(string, String::kLengthOffset));
   ThrowIf(ge, kIndexIsTooLarge, index, Operand(at));
 
-  li(at, Operand(Smi::FromInt(0)));
-  ThrowIf(lt, kIndexIsNegative, index, Operand(at));
+  ASSERT(Smi::FromInt(0) == 0);
+  ThrowIf(lt, kIndexIsNegative, index, Operand(zero_reg));
 
   SmiUntag(index, index);
 }