Change the x64 optimizing compiler to stop using the fixed context register rsi.

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 c23eb168978a1438032776cb1b453ce5a91e0b46..637a15fa755674aa32fe3eea4aeb23bad2714557 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -91,12 +91,11 @@ 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 kNumRegisters = 16;
-  static const int kNumAllocatableRegisters = 10;
+  static const int kNumAllocatableRegisters = 11;
 
   static int ToAllocationIndex(Register reg) {
     return kAllocationIndexByRegisterCode[reg.code()];
@@ -115,6 +114,7 @@ struct Register {
       "rbx",
       "rdx",
       "rcx",
+      "rsi",
       "rdi",
       "r8",
       "r9",