[VPlan] Assign custom opcodes to recipes not mapping to IR opcodes. (#162267)

We can perform CSE on recipes that do not directly map to Instruction
opcodes. One example is VPVectorPointerRecipe. Currently this is handled
by supporting them in ::canHandle, but currently that means that we
return std::nullopt from getOpcodeOrIntrinsicID() for it. This currently
only works, because the only case we return std::nullopt and perform CSE
is VPVectorPointerRecipe. But that does not work if we support more such
recipes, like VPPredInstPHIRecipe
(https://github.com/llvm/llvm-project/pull/162110).

To fix this, return a custom opcode from getOpcodeOrIntrinsicID for
recipes like VPVectorPointerRecipe, using the VPDefID after all regular
instruction opcodes.

PR: https://github.com/llvm/llvm-project/pull/162267

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