[InstCombine] Only propagate known leading zeros from udiv input to output.

Put in a conservatively correct estimate for now. Avoids miscompiling
clang in FDO mode. This is really tricky to trigger in reality as
basically all interesting cases will be folded away by computeKnownBits
earlier, I was unable to find a reasonably small test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@331975 91177308-0d34-0410-b5e6-96231b3b80d8