InstCombine: Teach icmp merging about the equivalence of bit tests and UGE/ULT with a power of 2.

This happens in bitfield code. While there reorganize the existing code
a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201176 91177308-0d34-0410-b5e6-96231b3b80d8