Auto merge of #116012 - cjgillot:gvn-const, r=oli-obk

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.](2767c4912e)"

r? `@oli-obk`

compiler/rustc_mir_transform/src/lib.rs

@@ -588,7 +588,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             // destroy the SSA property. It should still happen before const-propagation, so the
             // latter pass will leverage the created opportunities.
             &separate_const_switch::SeparateConstSwitch,
-            &const_prop::ConstProp,
             &gvn::GVN,
             &simplify::SimplifyLocals::AfterGVN,
             &dataflow_const_prop::DataflowConstProp,