Fixed crashes exposed though fuzzing.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index cafde512246b5a81f6f21e2b39e5ff04724ddc92..ba8ccc5cc995ceade12a62eb68048aa4b5a0d726 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -1732,17 +1732,14 @@ void LCodeGen::DoSeqStringSetChar(LSeqStringSetChar* instr) {
   Register string = ToRegister(instr->string());
 
   if (FLAG_debug_code) {
-    __ push(string);
-    __ movq(string, FieldOperand(string, HeapObject::kMapOffset));
-    __ movzxbq(string, FieldOperand(string, Map::kInstanceTypeOffset));
-
-    __ andb(string, Immediate(kStringRepresentationMask | kStringEncodingMask));
+    Register value = ToRegister(instr->value());
+    Register index = ToRegister(instr->index());
     static const uint32_t one_byte_seq_type = kSeqStringTag | kOneByteStringTag;
     static const uint32_t two_byte_seq_type = kSeqStringTag | kTwoByteStringTag;
-    __ cmpq(string, Immediate(encoding == String::ONE_BYTE_ENCODING
-                              ? one_byte_seq_type : two_byte_seq_type));
-    __ Check(equal, kUnexpectedStringType);
-    __ pop(string);
+    int encoding_mask =
+        instr->hydrogen()->encoding() == String::ONE_BYTE_ENCODING
+        ? one_byte_seq_type : two_byte_seq_type;
+    __ EmitSeqStringSetCharCheck(string, index, value, encoding_mask);
   }
 
   Operand operand = BuildSeqStringOperand(string, instr->index(), encoding);