[turbofan] Make pipeline.h independent of compiler.h.

src/compiler/pipeline.cc

Index: src/compiler/pipeline.cc
diff --git a/src/compiler/pipeline.cc b/src/compiler/pipeline.cc
index 01f55bce809939c591bf219b6906ca354ee23a3c..2e9332536245c56e333e75af3dd3a414dddc0b9d 100644
--- a/src/compiler/pipeline.cc
+++ b/src/compiler/pipeline.cc
@@ -1502,6 +1502,8 @@ void Pipeline::AllocateRegisters(const RegisterConfiguration* config,
   data->DeleteRegisterAllocationZone();
 }
 
+Isolate* Pipeline::isolate() const { return info()->isolate(); }
+
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8