[PATCH] [IR] Introduce a dereferenceable_xor_null(N) attribute.

[Sanjoy Das]

Hi hfinkel, rafael,

If a pointer is marked as dereferenceable_xor_null(N), LLVM assumes it
is exactly one of { null, dereferenceable(N) }.  This change only
introduces the attribute and adds a token test case for the `llvm-as` /
`llvm-dis`.  It does not hook up other parts of the optimizer to
actually exploit the attribute -- those changes will come later.

For pointers in address space 0, `dereferenceable(N)` is now exactly
equivalent to `dereferenceable_xor_null(N)` && `nonnull`.  For other
address spaces, `dereferenceable(N)` is potentially weaker than
`dereferenceable_xor_null(N)` && `nonnull` (since we could have a null
`dereferenceable(N)` pointer).

The motivating case for this change is Java (and other managed
languages), where pointers are either `null` or dereferenceable up to
some usually known-at-compile-time constant offset.

http://reviews.llvm.org/D8650

Files:
  docs/LangRef.rst
  include/llvm-c/Core.h
  include/llvm/Bitcode/LLVMBitCodes.h
  include/llvm/IR/Attributes.h
  include/llvm/IR/Function.h
  include/llvm/IR/Instructions.h
  lib/AsmParser/LLLexer.cpp
  lib/AsmParser/LLParser.cpp
  lib/AsmParser/LLParser.h
  lib/AsmParser/LLToken.h
  lib/Bitcode/Reader/BitcodeReader.cpp
  lib/Bitcode/Writer/BitcodeWriter.cpp
  lib/IR/AttributeImpl.h
  lib/IR/Attributes.cpp
  lib/IR/Function.cpp
  lib/IR/Instructions.cpp
  test/Assembler/deref-xor-null.ll

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