[llvm] [VPlan] Remove VPInstruction::Not. NFC (PR #222228)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I'm not sure about this one, personally I find `not %x` more readable than `xor %x, -1` in the output and I am not sure what we gain from removing it?

Having two canonical forms for `not` means the `m_Not` pattern matchers need to match two forms which isn't great for compile time. It also makes it difficult to improve the compile time of simplifyRecipes further, since we can't switch on the opcode when the same pattern can be matched on two separate opcodes. 

If we really want we could just adjust the printing for VPInstruction::Xor to print `not %x` for `xor %x, -1`. 

https://github.com/llvm/llvm-project/pull/222228