sbitmap.h: add operator const_sbitmap to auto_sbitmap

gcc/ChangeLog:
	* sbitmap.h (auto_sbitmap): Add operator const_sbitmap.

From-SVN: r280026

gcc/ChangeLog

+2020-01-08  David Malcolm  <dmalcolm@redhat.com>
+
+	* sbitmap.h (auto_sbitmap): Add operator const_sbitmap.
+
 2020-01-08  Jim Wilson  <jimw@sifive.com>
 
 	* config/riscv/riscv.c (riscv_legitimize_tls_address): Ifdef out

gcc/sbitmap.h

@@ -295,6 +295,7 @@ public:
 
   /* Allow calling sbitmap functions on our bitmap.  */
   operator sbitmap () { return m_bitmap; }
+  operator const_sbitmap () const { return m_bitmap; }
 
 private:
   /* Prevent making a copy that refers to our sbitmap.  */