Merge of r51073-51074 from use-diet branch.

Merge of r51073-51074 from use-diet branch.

Do not rely on std::swap<Use>, provide a (faster) member function instead.
This change is primarily necessitated by MSVC++'s incompatibility with
declaring std::swap<Use> to be a friend of Use.

Also contains some minor tweaks to Use inline functions,
to undo pointless changes that sneaked in with the last merge.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51078 91177308-0d34-0410-b5e6-96231b3b80d8