[InstCombine] Add general constant support to eq/ne icmp(add(X,C1),add(Y,C2))...

[InstCombine] Add general constant support to eq/ne icmp(add(X,C1),add(Y,C2)) -> icmp(add(X,C1-C2),Y) fold

A further extension for Issue #32161

For eq/ne comparisons - the sign mismatch and bounds constraints are redundant, so if the that fold fails, fallback and just fold the constants directly.

https://alive2.llvm.org/ce/z/cdodNQ

The loop rotation test change looks mostly benign - the backend doesn't seem to suffer? https://gcc.godbolt.org/z/dErMY78To

Differential Revision: https://reviews.llvm.org/D121551