Rollup merge of #122996 - RalfJung:simplify_branches, r=cjgillot

simplify_branches: add comment

I am not quite sure why this simplification is done here and not in InstSimplify but 🤷

r? `@cjgillot`

compiler/rustc_mir_transform/src/simplify_branches.rs

@@ -19,6 +19,7 @@ impl<'tcx> MirPass<'tcx> for SimplifyConstCondition {
         let param_env = tcx.param_env_reveal_all_normalized(body.source.def_id());
         'blocks: for block in body.basic_blocks_mut() {
             for stmt in block.statements.iter_mut() {
+                // Simplify `assume` of a known value: either a NOP or unreachable.
                 if let StatementKind::Intrinsic(box ref intrinsic) = stmt.kind
                     && let NonDivergingIntrinsic::Assume(discr) = intrinsic
                     && let Operand::Constant(ref c) = discr