MIPS: Fixing illegal use of at register

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index 5586ed1ab264e7dd1f1a68ed40a313ff71e370e3..9f0c8f5f3946d18fea7f01084fec0a9b0fdce911 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -4193,6 +4193,7 @@ void LCodeGen::DoStoreKeyedFixedDoubleArray(LStoreKeyed* instr) {
   DoubleRegister value = ToDoubleRegister(instr->value());
   Register elements = ToRegister(instr->elements());
   Register scratch = scratch0();
+  Register scratch_1 = scratch1();
   DoubleRegister double_scratch = double_scratch0();
   bool key_is_constant = instr->key()->IsConstantOperand();
   int base_offset = instr->base_offset();
@@ -4224,8 +4225,9 @@ void LCodeGen::DoStoreKeyedFixedDoubleArray(LStoreKeyed* instr) {
 
     // Only load canonical NaN if the comparison above set the overflow.
     __ bind(&is_nan);
-    __ LoadRoot(at, Heap::kNanValueRootIndex);
-    __ ldc1(double_scratch, FieldMemOperand(at, HeapNumber::kValueOffset));
+    __ LoadRoot(scratch_1, Heap::kNanValueRootIndex);
+    __ ldc1(double_scratch,
+            FieldMemOperand(scratch_1, HeapNumber::kValueOffset));
     __ sdc1(double_scratch, MemOperand(scratch, 0));
     __ Branch(&done);
   }