[llvm] Revert "[PowerPC] Add custom lowering for ssubo (#111748)" (PR #114672)

[Maryam Moghadas via llvm-commits]

maryammo wrote:

It appeared that the original [commit](https://github.com/llvm/llvm-project/commit/8a0cb9ac869334fd6c6bd6aad8408623a7ccd7f6) is causing the PPC bootstrap bot to fail: 
https://lab.llvm.org/buildbot/#/builders/76/builds/4050

The following tests failed: 
Clang::literals.cpp
Clang::enums-targets.cpp
Clang::constexpr-unsigned-high-bit.cpp
Clang::builtins-overflow.cpp

https://github.com/llvm/llvm-project/pull/114672