Revert revisions r234755, r234759, r234760

Revert revisions r234755, r234759, r234760

Revert "Remove default in fully-covered switch (to fix Clang -Werror -Wcovered-switch-default)"
Revert "R600: Add carry and borrow instructions. Use them to implement UADDO/USUBO"
Revert "LegalizeDAG: Try to use Overflow operations when expanding ADD/SUB"

Using overflow operations fails CodeGen/Generic/2011-07-07-ScheduleDAGCrash.ll
on hexagon, nvptx, and r600. Revert while I investigate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234768 91177308-0d34-0410-b5e6-96231b3b80d8