[InstCombine] NEW Baseline tests for InstCombine optimization to merge GEP instructio...

[InstCombine] NEW Baseline tests for InstCombine optimization to merge GEP instructions with constant indices

Splitted the merge constant-indexed GEP optimization into two smaller transformations: 1. Merging GEP of GEP if both are constant-indexed. 2. Swapping constant indexed GEP in a chain of (non-constant) GEP to the end, so that 1 can be applied repeatedly.
There is existing code to partially  handle transformation 1, but it only deals with limited cases

Unit tests are breaking down into two parts for the 2 transformations.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D125438