Revert rG1c9bec72 : [InstCombine] Fold (gep...

Revert rG1c9bec72 : [InstCombine] Fold (gep (oneuse(gep Ptr, Idx0)), Idx1) -> (gep Ptr, (add Idx0, Idx1)) (PR51069)

Reverted (manually due to merge conflicts) while regressions reported on PR51540 are investigated

As noticed on D106352, after we've folded "(select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0))" if the inner Ptr was also a (now one use) gep we could then merge the geps, using the sum of the indices instead.

I've limited this to basic 2-op geps - a more general case further down InstCombinerImpl.visitGetElementPtrInst doesn't have the one-use limitation but only creates the add if it can be created via SimplifyAddInst.

https://alive2.llvm.org/ce/z/f8pLfD (Thanks Roman!)

Differential Revision: https://reviews.llvm.org/D106450