Auto merge of #149366 - cjgillot:gvn-primitive, r=RalfJung

GVN: consider constants of primitive types as deterministic

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GVN separates MIR constants into deterministic and non-deterministic constants. Deterministic constants are defined here as: gives the exact same value each time it is evaluated.

This was mainly useful because of `ConstValue::Slice` that generated an extra `AllocId` each time it appeared in the MIR. That variant has been removed. This is still useful for valtrees that hold references, which generate a fresh `AllocId` for each evaluation.

This PR proposes to consider all constants of primitive type to be deterministic. If a constant of primitive type passes validation, then it does not contain provenance, so we have no risk of having a reference becoming different `AllocId`s. In particular, valtrees only are leaves.

r? @ghost for perf

Author: bors