Fixed crashes exposed though fuzzing.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index b911c48252eb440da9979350f174be5e9c22125c..0b2bf822e847e50f3d9c9c74c36e5a5a93428481 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1796,9 +1796,19 @@ LInstruction* LChunkBuilder::DoSeqStringGetChar(HSeqStringGetChar* instr) {
 
 LInstruction* LChunkBuilder::DoSeqStringSetChar(HSeqStringSetChar* instr) {
   LOperand* string = UseRegisterAtStart(instr->string());
-  LOperand* index = UseRegisterOrConstantAtStart(instr->index());
-  LOperand* value = UseRegisterOrConstantAtStart(instr->value());
-  return new(zone()) LSeqStringSetChar(string, index, value);
+  LOperand* index = FLAG_debug_code
+      ? UseRegisterAtStart(instr->index())
+      : UseRegisterOrConstantAtStart(instr->index());
+  LOperand* value = FLAG_debug_code
+      ? UseRegisterAtStart(instr->value())
+      : UseRegisterOrConstantAtStart(instr->value());
+  LOperand* context = FLAG_debug_code ? UseFixed(instr->context(), rsi) : NULL;
+  LInstruction* result = new(zone()) LSeqStringSetChar(context, string,
+                                                       index, value);
+  if (FLAG_debug_code) {
+    result = MarkAsCall(result, instr);
+  }
+  return result;
 }