[CVP] Infer nneg on existing zext (#72052)

This patch infers `nneg` flags for existing zext instructions in CVP.
After https://github.com/llvm/llvm-project/pull/71534 and this patch, we
can drop `zext -> zext nneg` transform in `RISCVCodeGenPrepare`:


https://github.com/llvm/llvm-project/blob/40671bbdefb6ff83e2685576a3cb041b62f25bbe/llvm/lib/Target/RISCV/RISCVCodeGenPrepare.cpp#L74-L83

This is an alternative to #72049.