[TOY] Extend GVN to perform local value numbering.

[cjgillot]

This PR is more a toy than anything else, but I still think the implementation is sound.

Current GVN MIR opt can be easily extended to track values that change inside a basic block. This PR attempts that.

r? @ghost for perf

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [mir-opt] tests/mir-opt/unusual_item_types.rs ... ok

failures:

---- [mir-opt] tests/mir-opt/inline_coroutine_body.rs stdout ----
256 +         _50 = &mut (_19.0: &mut std::future::Ready<()>);
257 +         _47 = copy (_19.0: &mut std::future::Ready<()>);
258 +         StorageDead(_50);
- +         _49 = Pin::<&mut std::future::Ready<()>> { pointer: copy _47 };
+ +         _49 = copy _19;
260 +         StorageDead(_49);
261 +         _46 = &mut ((*_47).0: std::option::Option<()>);
262 +         StorageLive(_51);


thread '[mir-opt] tests/mir-opt/inline_coroutine_body.rs' panicked at src/tools/compiletest/src/runtest/mir_opt.rs:84:21:
Actual MIR output differs from expected MIR output /checkout/tests/mir-opt/inline_coroutine_body.run2-{closure#0}.Inline.panic-unwind.diff
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    [mir-opt] tests/mir-opt/inline_coroutine_body.rs