[llvm] [CodeGen] Check for no VN before def to catch potential split components (PR #191141)

[via llvm-commits]

sc-clulzze wrote:

About the failing test. I believe that initial MIR is invalid because it has this multiple connected components for %4. So when this pass gets LiveIntervals analysis after this fix, interval for %4 is split into 2 different register and optimization is not being applied.
As I understand HexagonCopyHoisting does not use LIS, maybe we should remove LIS from addRequired and addPreserved then? This would fix the issue. @quic-pmacmurr @mikaelholmen Could you comment on this please?

https://github.com/llvm/llvm-project/pull/191141