Use register allocator for context on x64.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 508c62211237f61d7f78198db4730e9848718064..680dcadc2041c68e76f95ec73fcc3854e010c09c 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -91,11 +91,10 @@ struct Register {
   // The non-allocatable registers are:
   //  rsp - stack pointer
   //  rbp - frame pointer
-  //  rsi - context register
   //  r10 - fixed scratch register
   //  r12 - smi constant register
   //  r13 - root register
-  static const int kMaxNumAllocatableRegisters = 10;
+  static const int kMaxNumAllocatableRegisters = 11;
   static int NumAllocatableRegisters() {
     return kMaxNumAllocatableRegisters;
   }
@@ -118,6 +117,7 @@ struct Register {
       "rbx",
       "rdx",
       "rcx",
+      "rsi",
       "rdi",
       "r8",
       "r9",