Zig redefines @bitCast, boosts LLVM

This update demonstrates how aligning compiler backends with formal language specifications can resolve long-standing optimization bugs while simultaneously delivering free performance. By abandoning memory-based casts for a logical bit model, Zig also paves the way for cleaner compiler code legalization.

• –Moving `@bitCast` to logic-based bit reinterpretation makes the operation endian-agnostic on aggregates (like arrays and vectors) and matches previous little-endian behavior.
• –Lowering arbitrary bit-width integers (e.g., `u4`, `i13`) to memory as zero- or sign-extended ABI-sized types (`i8`, `i16`, etc.) avoids historically buggy LLVM IR paths.
• –The backend change resulted in a ~5% performance improvement for the self-hosted Zig compiler by restoring optimizations LLVM previously missed.
• –Leveraging the compiler's `Legalize` pass allows backend code (LLVM, C, etc.) to offload complex casting logic to a shared step, simplifying platform-specific implementations.
• –The change also incorporates accepted proposals such as disallowing pointer vector bitcasts and officially supporting `@bitCast` on enums.