[llvm] [InstCombine] Simplify `(add/sub (sub/add) (sub/add))` irrelivant of use-count (PR #105866)

[via llvm-commits]

goldsteinn wrote:

NB regarding if this patch exists, saw changes in:

./lib/Demangle/MicrosoftDemangle.cpp
./lib/Target/ARM/ARMConstantIslandPass.cpp
./lib/Target/ARM/ARMISelDAGToDAG.cpp
./lib/Target/Mips/MipsConstantIslandPass.cpp
./lib/Transforms/Scalar/DeadStoreElimination.cpp
./lib/Transforms/Vectorize/SLPVectorizer.cpp
./tools/clang/lib/Edit/EditedSource.cpp
./tools/clang/lib/Format/BreakableToken.cpp

when bootstrapping LLVM (changes in final output of O3 pipeline).

https://github.com/llvm/llvm-project/pull/105866