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

[cjgillot]

Based on #148151

This PR fully removes the MIR Rvalue::NullaryOp. After #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.

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to constck

cc @fee1-dead

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

[rustbot]

r? @lcnr

rustbot has assigned @lcnr.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer