[APInt] Implement tcDecrement as a counterpart to tcIncrement. This is for use in APFloat IEEE-754R 2008 nextUp/nextDown function.

rdar://13852078

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182801 91177308-0d34-0410-b5e6-96231b3b80d8