[turbofan] Refactor the OSR environment merging to create an explicit environment.

src/compiler/ast-graph-builder.h

Index: src/compiler/ast-graph-builder.h
diff --git a/src/compiler/ast-graph-builder.h b/src/compiler/ast-graph-builder.h
index edef1c08329ca4b32901377f5833f560e5d45b6e..27f2c9b4c61346203787cc8ab198f24f642bc05b 100644
--- a/src/compiler/ast-graph-builder.h
+++ b/src/compiler/ast-graph-builder.h
@@ -579,6 +579,11 @@ class AstGraphBuilder::Environment : public ZoneObject {
   // Copies this environment at a loop header control-flow point.
   Environment* CopyForLoop(BitVector* assigned, bool is_osr = false);
 
+  // Copies this environment for Osr entry. This only produces environment
+  // of the right shape, the caller is responsible for filling in the right
+  // values and dependencies.
+  Environment* CopyForOsrEntry();
+
  private:
   AstGraphBuilder* builder_;
   int parameters_count_;
@@ -608,7 +613,8 @@ class AstGraphBuilder::Environment : public ZoneObject {
   bool IsLivenessBlockConsistent();
 
   // Prepare environment to be used as loop header.
-  void PrepareForLoop(BitVector* assigned, bool is_osr = false);
+  void PrepareForLoop(BitVector* assigned);
+  void PrepareForOsrEntry();
 };
 
 }  // namespace compiler