Fix PR21694. r219517 added a use of SCEV divide in HowFarToZero computation.  This divide can produce incorrect results as we are using an unsigned divide for what should be a modular divide.  This change reverts back to a more conservative computation using trailing zeros.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223974 91177308-0d34-0410-b5e6-96231b3b80d8