[llvm] [InstCombine] Transform `vector.reduce.add` and `splat` into multiplication (PR #161020)

[Gábor Spaits via llvm-commits]

spaits wrote:

There is one lldb failure on Linux. I think that is just a flaky test case, which isn't caused by this PR. I will retrigger the CI.

https://github.com/llvm/llvm-project/pull/161020