[PATCH] Add a dereferencable attribute

[JF Bastien]

Looks good to me, besides two small comments.

================
Comment at: include/llvm/IR/Attributes.h:91
@@ -90,2 +90,3 @@
     NonNull,               ///< Pointer is known to be not null
+    Dereferenceable,       ///< Pointer is known to be dereferenceable
     NoRedZone,             ///< Disable redzone
----------------
You'll make some people sad (not PNaCl, but Renderscript and others) by adding the attribute here in the enum instead of at the end, since it changes bitcode slightly.

================
Comment at: lib/IR/Attributes.cpp:93
@@ +92,3 @@
+                                                uint64_t Bytes) {
+  assert(Bytes && "Bytes must be positive.");
+  return get(Context, Dereferenceable, Bytes);
----------------
Math bikeshed: "non-zero" seems better than "positive".

http://reviews.llvm.org/D4449