Fix warnings on Win64.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 474d4b861356fc3e253f32b8c9c29b34d2178a7f..d2a979c3eda318a8bdccf83a36cdba1c5ae5c90a 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -1131,7 +1131,7 @@ class Assembler : public Malloced {
   void RecordStatementPosition(int pos);
   void WriteRecordedPositions();
 
-  int pc_offset() const  { return pc_ - buffer_; }
+  int pc_offset() const  { return static_cast<int>(pc_ - buffer_); }
   int current_statement_position() const { return current_statement_position_; }
   int current_position() const  { return current_position_; }
 
@@ -1143,7 +1143,9 @@ class Assembler : public Malloced {
   }
 
   // Get the number of bytes available in the buffer.
-  inline int available_space() const { return reloc_info_writer.pos() - pc_; }
+  inline int available_space() const {
+    return static_cast<int>(reloc_info_writer.pos() - pc_);
+  }
 
   // Avoid overflows for displacements etc.
   static const int kMaximalBufferSize = 512*MB;