[SeparateConstOffsetFromGEP] Fixed a bug in rebuilding OR expressions

The two operands of the new OR expression should be NextInChain and TheOther
instead of the two original operands.

Added a regression test in split-gep.ll.

Hao Liu reported this bug, and provded the test case and an initial patch.
Thanks! 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220615 91177308-0d34-0410-b5e6-96231b3b80d8