Make blockaddress(@func, null) be valid, and make 'deleting a basic

Make blockaddress(@func, null) be valid, and make 'deleting a basic
block with a blockaddress still referring to it' replace the invalid
blockaddress with a new blockaddress(@func, null) instead of a
inttoptr(1).

This changes the bitcode encoding format, and still needs codegen
support (this should produce a non-zero value, referring to the entry
block of the function would also be quite reasonable).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85678 91177308-0d34-0410-b5e6-96231b3b80d8