APInt: udivrem should use machine instructions for single-word APInts

This mirrors the behavior of APInt::udiv and APInt::urem.  Some
architectures, like X86, have a single instruction which can compute
both division and remainder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224217 91177308-0d34-0410-b5e6-96231b3b80d8