[llvm] [LV] Handle partial reductions with predication (PR #194859)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Could a number of these checks be folded into the existing `m_Select` checks with https://github.com/llvm/llvm-project/pull/194729? As IIUC, this is handling the case where the blend is a select.

https://github.com/llvm/llvm-project/pull/194859