Reland r262337 "calculate builtin_object_size if arg is a removable pointer"

Original commit message:
 calculate builtin_object_size if argument is a removable pointer

 This patch fixes calculating correct value for builtin_object_size function
 when pointer is used only in builtin_object_size function call and never
 after that.

 Patch by Strahinja Petrovic.

 Differential Revision: http://reviews.llvm.org/D17337

Reland the original change with a small modification (first do a null check
and then do the cast) to satisfy ubsan.

llvm-svn: 263011