Fixed crashes exposed though fuzzing.

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index 71ce8f11a99afc680d80eddc0943b7b01700ce9b..a6644af5ef78a605a9beb003dfd93228339dba32 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -1906,10 +1906,13 @@ 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);
 }