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Implement constant propagation on top of MIR SSA analysis This implements the idea I proposed in https://github.com/rust-lang/rust/pull/110719#issuecomment-1718324700 Based on https://github.com/rust-lang/rust/pull/109597 The value numbering "GVN" pass formulates each rvalue that appears in MIR with an abstract form (the `Value` enum), and assigns an integer `VnIndex` to each. This abstract form can be used to deduplicate values, reusing an earlier local that holds the same value instead of recomputing. This part is proposed in #109597. From this abstract representation, we can perform more involved simplifications, for example in https://github.com/rust-lang/rust/pull/111344. With the abstract representation `Value`, we can also attempt to evaluate each to a constant using the interpreter. This builds a `VnIndex -> OpTy` map. From this map, we can opportunistically replace an operand or a rvalue with a constant if their value has an associated `OpTy`. The most relevant commit is [Evaluated computed values to constants.](https://github.com/rust-lang/rust/commit/2767c4912ea249c2f613a9cedcd6c13ea1237e54)" r? `@oli-obk`