Update V8 to r6190 as required by WebKit r75315

Change-Id: I0b2f598e4d8748df417ad350fc47a1c465ad1fef
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 1df9b47..70a3dab 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -1110,7 +1110,7 @@
 
 void MacroAssembler::SmiAndConstant(Register dst, Register src, Smi* constant) {
   if (constant->value() == 0) {
-    xor_(dst, dst);
+    Set(dst, 0);
   } else if (dst.is(src)) {
     ASSERT(!dst.is(kScratchRegister));
     Register constant_reg = GetSmiConstant(constant);
@@ -1605,7 +1605,7 @@
 #ifdef ENABLE_DEBUGGER_SUPPORT
 void MacroAssembler::DebugBreak() {
   ASSERT(allow_stub_calls());
-  xor_(rax, rax);  // no arguments
+  Set(rax, 0);  // No arguments.
   movq(rbx, ExternalReference(Runtime::kDebugBreak));
   CEntryStub ces(1);
   Call(ces.GetCode(), RelocInfo::DEBUG_BREAK);