[llvm] [InstCombine] Decompose constant xor operand if possible (PR #147599)

[Sumanth Gundapaneni via llvm-commits]

sgundapa wrote:

Thanks for reviewing @nikic. I have originally attempted to do this here https://github.com/llvm/llvm-project/pull/135788, and was directed to give a try in InstCombine. Would it make sense if I restrict the transformation to just GEPs. What I am really after is folding these xor computations in to GEPs, so that I do not have to recompute address for each of the loads (referring to the attached test in the patch).  

https://github.com/llvm/llvm-project/pull/147599