Take the next steps in making SDUse more consistent with LLVM Use, and

Take the next steps in making SDUse more consistent with LLVM Use, and
tidy up SDUse and related code.
 - Replace the operator= member functions with a set method, like
   LLVM Use has, and variants setInitial and setNode, which take
   care up updating use lists, like LLVM Use's does. This simplifies
   code that calls these functions.
 - getSDValue() is renamed to get(), as in LLVM Use, though most
   places can either use the implicit conversion to SDValue or the
   convenience functions instead.
 - Fix some more node vs. value terminology issues.

Also, eliminate the one remaining use of SDOperandPtr, and
SDOperandPtr itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62995 91177308-0d34-0410-b5e6-96231b3b80d8