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
Avoid caching the MachineFunction, we don't use it outside of
runOnMachineFunction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219847 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Fix bug where immediates were being used in DS addr operands

The SelectDS1Addr1Offset complex pattern always tries to store constant
lds pointers in the offset operand and store a zero value in the addr operand.
Since the addr operand does not accept immediates, the zero value
needs to first be copied to a register.

This newly created zero value will not go through normal instruction
selection, so we need to manually insert a V_MOV_B32_e32 in the complex
pattern.

This bug was hidden by the fact that if there was another zero value
in the DAG that had not been selected yet, then the CSE done by the DAG
would use the unselected node for the addr operand rather than the one
that was just created.  This would lead to the zero value being selected
and the DAG automatically inserting a V_MOV_B32_e32 instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219848 91177308-0d34-0410-b5e6-96231b3b80d8