[llvm] [VPlan] Use bind_front to improve code in PhiAccessors (NFC) (PR #190167)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

bind_front avoids the creation of a std::function, and is actually more efficient. The auto changes were necessitated by the bind_front change.

https://github.com/llvm/llvm-project/pull/190167