[llvm] [PowerPC] Use the same lowering rule for vector rounding instructions (PR #166307)

[Lei Huang via llvm-commits]

lei137 wrote:

Since I had it already: [[PowerPC][NFC] auto gen checks vec rounding tests](https://github.com/llvm/llvm-project/pull/166435)

https://github.com/llvm/llvm-project/pull/166307