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

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 2ee506d7c3021d8f61cca1788ee0f228d75aec19..7ee1b0c9c9a1059aed65986a6baf44d8f54b94dc 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -261,11 +261,6 @@ void MacroAssembler::RecordWrite(Register object,
                                  int offset,
                                  Register value,
                                  Register index) {
-  // The compiled code assumes that record write doesn't change the
-  // context register, so we check that none of the clobbered
-  // registers are rsi.
-  ASSERT(!object.is(rsi) && !value.is(rsi) && !index.is(rsi));
-
   // First, check if a write barrier is even needed. The tests below
   // catch stores of smis and stores into the young generation.
   Label done;
@@ -290,11 +285,6 @@ void MacroAssembler::RecordWrite(Register object,
 void MacroAssembler::RecordWrite(Register object,
                                  Register address,
                                  Register value) {
-  // The compiled code assumes that record write doesn't change the
-  // context register, so we check that none of the clobbered
-  // registers are rsi.
-  ASSERT(!object.is(rsi) && !value.is(rsi) && !address.is(rsi));
-
   // First, check if a write barrier is even needed. The tests below
   // catch stores of smis and stores into the young generation.
   Label done;