Fix assert in ObjCARC optimizer when deleting retainBlock of null or undef.

The caller to EraseInstruction had this conditional:

    // ARC calls with null are no-ops. Delete them.
    if (IsNullOrUndef(Arg))

but the assert inside EraseInstruction only allowed ConstantPointerNull and not
undef or bitcasts.

This adds support for both of these cases.

rdar://problem/47003805

llvm-svn: 350261