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

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 745fdaeb8f2f8572bea2d5843d2fd66e030f9fce..399bedab88186da60252b1c6889cb1dd59992279 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -201,11 +201,11 @@ void RelocInfo::PatchCode(byte* instructions, int instruction_count) {
 
 const int Register::kRegisterCodeByAllocationIndex[kNumAllocatableRegisters] = {
   // rax, rbx, rdx, rcx, rdi, r8, r9, r11, r14, r15
-  0, 3, 2, 1, 7, 8, 9, 11, 14, 15
+  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
 };