Optimize MIR CFG generation for chained logical operators (#83623)

[sunny026]

Fixes #83623

Description

This PR resolves the issue where chained logical operators (&& and ||) evaluated into a boolean destination generated suboptimal MIR. Previously, the AST lowering in compiler/rustc_mir_build/src/builder/expr/into.rs recursively invoked expr_into_dest, which allocated duplicate destination = false basic blocks for every operator in the chain. This fractured control flow prevented LLVM's mem2reg pass from recognizing the pattern and blocked auto-vectorization.

Changes

• Modified ExprKind::LogicalOp lowering inside into.rs to route the entire expression through then_else_break.
• Because then_else_break optimally shares if/then/else scopes without creating temporary booleans, this natively collapses all short-circuiting branches into a single unified success block and a single unified failure block.

Result

LLVM can now instantly convert the unified goto failures into SSA Phi nodes, enabling seamless SIMD vectorization for constructs like #[derive(PartialEq)] on large structs!

[rustbot]

Some changes occurred in coverage instrumentation.

cc @Zalathar

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) some time within the next two weeks.

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• Owners of files modified in this PR: compiler, mir
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[cjgillot]

How does this fix the bug? From a glance it looks like it's making it worse. Do you have a test?

[sunny026]

How does this fix the bug? From a glance it looks like it's making it worse. Do you have a test?

Yes tested

[cjgillot]

Do you mind committing in tests/mir-opt it so we can see what changes in practice?