Cranelift: Fix select condition harvesting (#5662)

Souper requires an `i1` condition value, we don't and will implicitly check
against 0. We were truncating conditions but should actually be doing the
comparison against `0`.

cranelift/codegen/src/souper_harvest.rs

@@ -387,7 +387,8 @@ fn harvest_candidate_lhs(
                         let a = arg(allocs, 0);
 
                         // While Cranelift allows any width condition for
-                        // `select`, Souper requires an `i1`.
+                        // `select` and checks it against `0`, Souper requires
+                        // an `i1`. So insert a `ne %x, 0` as needed.
                         let a = match a {
                             ast::Operand::Value(id) => match lhs.get_value(id).r#type {
                                 Some(ast::Type { width: 1 }) => a,
@@ -395,7 +396,14 @@ fn harvest_candidate_lhs(
                                     .assignment(
                                         None,
                                         Some(ast::Type { width: 1 }),
-                                        ast::Instruction::Trunc { a },
+                                        ast::Instruction::Ne {
+                                            a,
+                                            b: ast::Constant {
+                                                value: 0,
+                                                r#type: None,
+                                            }
+                                            .into(),
+                                        },
                                         vec![],
                                     )
                                     .into(),