Use register allocator for context on x64.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index dcb9fa562159b8e33f2cfff8b7ffcf205c57b77e..8e5b4213a78b8f1e4744de81fb44ffbbe737f20a 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -140,12 +140,12 @@ void RelocInfo::PatchCode(byte* instructions, int instruction_count) {
 
 const int
     Register::kRegisterCodeByAllocationIndex[kMaxNumAllocatableRegisters] = {
-  // rax, rbx, rdx, rcx, rdi, r8, r9, r11, r14, r15
-  0, 3, 2, 1, 7, 8, 9, 11, 14, 15
+  // rax, rbx, rdx, rcx, rsi, rdi, r8, r9, r11, r14, r15
+  0, 3, 2, 1, 6, 7, 8, 9, 11, 14, 15
 };
 
 const int Register::kAllocationIndexByRegisterCode[kNumRegisters] = {
-  0, 3, 2, 1, -1, -1, -1, 4, 5, 6, -1, 7, -1, -1, 8, 9
+  0, 3, 2, 1, -1, -1, 4, 5, 6, 7, -1, 8, -1, -1, 9, 10
 };