Auto merge of #148766 - cjgillot:mir-const-runtime-checks, r=RalfJung,saethlin

Replace Rvalue::NullaryOp by a variant in mir::Operand.

Based on rust-lang/rust#148151

This PR fully removes the MIR `Rvalue::NullaryOp`. After rust-lang/rust#148151, it was only useful for runtime checks like `ub_checks`, `contract_checks` and `overflow_checks`.

These are "runtime" checks, boolean constants that may only be `true` in codegen. It depends on a rustc flag passed to codegen, so we need to represent those flags cross-crate.

This PR replaces those runtime checks by special variants in MIR `ConstValue`. This allows code that expects constants to manipulate those as such, even if we may not always be able to evaluate them to actual scalars.

Author: bors