[llvm] [InstCombine] Generalize zext(add X, -C) + C folding (PR #191723)

[via llvm-commits]

Maiowaa wrote:

Updated in v2:

Restored accidentally deleted FMF propagation block in foldFNegIntoConstant
Moved test from separate zext-add-general.ll to add.ll
Fixed clang-format: split long if condition across two lines

https://github.com/llvm/llvm-project/pull/191723