[llvm] [VPlan] Simplify select c, x, x -> x, add m_Deferred (PR #133731)

[Luke Lau via llvm-commits]

lukel97 wrote:

Actually I've just noticed the combine above can use m_Deferred. I've split off the m_Deferred bit into https://github.com/llvm/llvm-project/pull/133736 to keep it NFC

https://github.com/llvm/llvm-project/pull/133731