Disentangle assembler from isolate.

src/s390/assembler-s390.h

Index: src/s390/assembler-s390.h
diff --git a/src/s390/assembler-s390.h b/src/s390/assembler-s390.h
index e6185d5c09eae41248c80894a513cae0b67e45fa..311ecfcab0bacb375a75b951b9541960a36b091a 100644
--- a/src/s390/assembler-s390.h
+++ b/src/s390/assembler-s390.h
@@ -397,7 +397,9 @@ class Assembler : public AssemblerBase {
   // for code generation and assumes its size to be buffer_size. If the buffer
   // is too small, a fatal error occurs. No deallocation of the buffer is done
   // upon destruction of the assembler.
-  Assembler(Isolate* isolate, void* buffer, int buffer_size);
+  Assembler(Isolate* isolate, void* buffer, int buffer_size)
+      : Assembler(IsolateData(isolate), buffer, buffer_size) {}
+  Assembler(IsolateData isolate_data, void* buffer, int buffer_size);
   virtual ~Assembler() {}
 
   // GetCode emits any pending (non-emitted) code and fills the descriptor
@@ -441,6 +443,7 @@ class Assembler : public AssemblerBase {
   void load_label_offset(Register r1, Label* L);
 
   // Read/Modify the code target address in the branch/call instruction at pc.
+  // The isolate argument is unused (and may be nullptr) when skipping flushing.
   INLINE(static Address target_address_at(Address pc, Address constant_pool));
   INLINE(static void set_target_address_at(
       Isolate* isolate, Address pc, Address constant_pool, Address target,