[llvm] [CodeGen] Discard oracle functions during ISel translation (PR #188017)

[Ralf Jung via llvm-commits]

RalfJung wrote:

Forcing the oracle to be truly only used as an oracle makes sense to me, thanks.

(When not running with llubi, we could even entirely skip generating the oracle function. What's important it to explicitly encode and respect the data dependencies of the oracle -- which variables are needed to compute which bytes will actually be read. So it's enough to encode "the oracle has this signature and these are the values we are passing to it". That's the only part that should affect other optimization passes.)

https://github.com/llvm/llvm-project/pull/188017