MIPS: Fix for Remove unsafe EmitLoadRegister usage in AddI/SubI for constant right operand.

Fix a missed addiu->Addu, and a bad default use of 'at' register in https://codereview.chromium.org/1185143002/

TEST=test/mjsunit/regress/regress-500176
BUG=chromium:500176
LOG=N

Review URL: https://codereview.chromium.org/1187063002

Cr-Commit-Position: refs/heads/master@{#29034}

src/mips/lithium-codegen-mips.cc

@@ -1681,8 +1681,7 @@ void LCodeGen::DoSubI(LSubI* instr) {
     } else {
       DCHECK(right->IsRegister() || right->IsConstantOperand());
       __ SubuAndCheckForOverflow(ToRegister(result), ToRegister(left),
-                                 ToOperand(right),
-                                 overflow);  // Reg at also used as scratch.
+                                 ToOperand(right), overflow, scratch);
     }
     DeoptimizeIf(lt, instr, Deoptimizer::kOverflow, overflow,
                  Operand(zero_reg));
@@ -1878,8 +1877,7 @@ void LCodeGen::DoAddI(LAddI* instr) {
     } else {
       DCHECK(right->IsRegister() || right->IsConstantOperand());
       __ AdduAndCheckForOverflow(ToRegister(result), ToRegister(left),
-                                 ToOperand(right),
-                                 overflow);  // Reg at also used as scratch.
+                                 ToOperand(right), overflow, scratch);
     }
     DeoptimizeIf(lt, instr, Deoptimizer::kOverflow, overflow,
                  Operand(zero_reg));

src/mips/macro-assembler-mips.cc

@@ -4459,7 +4459,7 @@ void MacroAssembler::AdduAndCheckForOverflow(Register dst, Register left,
       xor_(overflow_dst, dst, t9);
       and_(overflow_dst, overflow_dst, scratch);
     } else {
-      addiu(dst, left, right.immediate());
+      Addu(dst, left, right.immediate());
       xor_(overflow_dst, dst, left);
       // Load right since xori takes uint16 as immediate.
       Addu(t9, zero_reg, right);