MIPS: Fixed crashes exposed though fuzzing.

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index 09bae598db9e738be2cb24a44c37b6bae3c2da50..fb816273547058d7e6a829e5e0dab64406df8429 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.cc
@@ -1826,10 +1826,13 @@ LInstruction* LChunkBuilder::DoSeqStringGetChar(HSeqStringGetChar* instr) {
 
 
 LInstruction* LChunkBuilder::DoSeqStringSetChar(HSeqStringSetChar* instr) {
-  LOperand* string = UseRegister(instr->string());
-  LOperand* index = UseRegisterOrConstant(instr->index());
-  LOperand* value = UseRegister(instr->value());
-  return new(zone()) LSeqStringSetChar(string, index, value);
+  LOperand* string = UseRegisterAtStart(instr->string());
+  LOperand* index = FLAG_debug_code
+      ? UseRegisterAtStart(instr->index())
+      : UseRegisterOrConstantAtStart(instr->index());
+  LOperand* value = UseRegisterAtStart(instr->value());
+  LOperand* context = FLAG_debug_code ? UseFixed(instr->context(), cp) : NULL;
+  return new(zone()) LSeqStringSetChar(context, string, index, value);
 }