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

[Lei Huang via llvm-commits]

https://github.com/lei137 approved this pull request.

I see the vsx instruction generated is not as good as the altivec ones but that seem to be an exisiting issue and is not caused by this patch. LGTM thx.

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