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. 

llvm-svn: 209670