Distribute sext/zext to the operands of and/or/xor

This is an enhancement to SeparateConstOffsetFromGEP. With this patch, we can
extract a constant offset from "s/zext and/or/xor A, B".

Added a new test @ext_or to verify this enhancement.

Refactoring the code, I also extracted some common logic to function
Distributable. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209670 91177308-0d34-0410-b5e6-96231b3b80d8