Update V8 to r6190 as required by WebKit r75315

Change-Id: I0b2f598e4d8748df417ad350fc47a1c465ad1fef
diff --git a/src/arm/assembler-arm.h b/src/arm/assembler-arm.h
index 36f7507..cd7f07f 100644
--- a/src/arm/assembler-arm.h
+++ b/src/arm/assembler-arm.h
@@ -66,13 +66,14 @@
 // such that we use an enum in optimized mode, and the struct in debug
 // mode. This way we get the compile-time error checking in debug mode
 // and best performance in optimized code.
-//
+
 // Core register
 struct Register {
   static const int kNumRegisters = 16;
   static const int kNumAllocatableRegisters = 8;
 
   static int ToAllocationIndex(Register reg) {
+    ASSERT(reg.code() < kNumAllocatableRegisters);
     return reg.code();
   }
 
@@ -132,7 +133,7 @@
 const Register r6  = {  6 };
 const Register r7  = {  7 };
 const Register r8  = {  8 };  // Used as context register.
-const Register r9  = {  9 };
+const Register r9  = {  9 };  // Used as lithium codegen scratch register.
 const Register r10 = { 10 };  // Used as roots register.
 const Register fp  = { 11 };
 const Register ip  = { 12 };