Replace incorrect "#ifdef DEBUG" with "#ifndef NDEBUG".

The former is simply wrong -- the code will either never be used or will
always be used, rather than being dependent upon whether it's built with
debug assertions enabled.

The macro DEBUG isn't ever set by the llvm build system. But, the macro
DEBUG(X) is defined (unconditionally) if you happen to include
llvm/Support/Debug.h.

The code in Value.h which was erroneously protected by the #ifdef DEBUG
didn't even compile -- you can't cast<> from an LLVMOpaqueValue
directly. Fortunately, it was never invoked, as Core.cpp included
Value.h before Debug.h.

The conditionalized code in AArch64CollectLOH.cpp was previously always
used, as it includes Debug.h.

llvm-svn: 280056