IR: Cleanup comments for Value, User, and MDNode

A follow-up commit will modify the memory-layout of `Value`, `User`, and
`MDNode`.  First fix the comments to be doxygen-friendly (and to follow
the coding standards).

  - Use "\brief" instead of "repeatedName -".
  - Add a brief intro where it was missing.
  - Remove duplicated comments from source files (and a couple of
    noisy/trivial comments altogether).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219844 91177308-0d34-0410-b5e6-96231b3b80d8
IR: Move NumOperands from User to Value, NFC

Store `User::NumOperands` (and `MDNode::NumOperands`) in `Value`.

On 64-bit host architectures, this reduces `sizeof(User)` and all
subclasses by 8, and has no effect on `sizeof(Value)` (or, incidentally,
on `sizeof(MDNode)`).

On 32-bit host architectures, this increases `sizeof(Value)` by 4.
However, it has no effect on `sizeof(User)` and `sizeof(MDNode)`, so the
only concrete subclasses of `Value` that actually see the increase are
`BasicBlock`, `Argument`, `InlineAsm`, and `MDString`.  Moreover, I'll
be shocked and confused if this causes a tangible memory regression.

This has no functionality change (other than memory footprint).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219845 91177308-0d34-0410-b5e6-96231b3b80d8
Wrong attribute.  LLVM_ATTRIBUTE_UNUSED not LLVM_ATTRIBUTE_USED

This original fix for the build break was correct.  LLVM_ATTRIBUTE_USED
removes the warning message because it keeps the function in the object
file.  LLVM_ATTRIBUTE_UNUSED indicates that it may or may not be used
depending on build settings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219846 91177308-0d34-0410-b5e6-96231b3b80d8