[DAGCombine][ARM] x ==/!= c -> (x - c) ==/!= 0 iff '-c' can be folded into the x node.

Summary:
This fold, helps recover from the rest of the D62266 ARM regressions.
https://rise4fun.com/Alive/TvpC

Note that while the fold is quite flexible, i've restricted it
to the single interesting pattern at the moment.

Reviewers: efriedma, craig.topper, spatel, RKSimon, deadalnix

Reviewed By: deadalnix

Subscribers: javed.absar, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62450