fix: Update databus in flattening (#6063)

# Description

## Problem\*

Resolves #5705

## Summary\*

We were using an inserter in flattening but forgot to update the databus

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/ssa/ir/function_inserter.rs

@@ -102,6 +102,14 @@ impl<'f> FunctionInserter<'f> {
         self.function.dfg[block].set_terminator(terminator);
     }
 
+    /// Maps the data bus in place, replacing any ValueId in the data bus with the
+    /// resolved version of that value id from this FunctionInserter's internal value mapping.
+    pub(crate) fn map_data_bus_in_place(&mut self) {
+        let data_bus = self.function.dfg.data_bus.clone();
+        let data_bus = data_bus.map_values(|value| self.resolve(value));
+        self.function.dfg.data_bus = data_bus;
+    }
+
     /// Push a new instruction to the given block and return its new InstructionId.
     /// If the instruction was simplified out of the program, None is returned.
     pub(crate) fn push_instruction(

compiler/noirc_evaluator/src/ssa/opt/flatten_cfg.rs

@@ -289,6 +289,7 @@ impl<'f> Context<'f> {
                 }
             }
         }
+        self.inserter.map_data_bus_in_place();
     }
 
     /// Returns the updated condition so that

compiler/noirc_evaluator/src/ssa/opt/mem2reg.rs

@@ -436,8 +436,7 @@ impl<'f> PerFunctionContext<'f> {
     }
 
     fn update_data_bus(&mut self) {
-        let databus = self.inserter.function.dfg.data_bus.clone();
-        self.inserter.function.dfg.data_bus = databus.map_values(|t| self.inserter.resolve(t));
+        self.inserter.map_data_bus_in_place();
     }
 
     fn handle_terminator(&mut self, block: BasicBlockId, references: &mut Block) {

test_programs/compile_success_no_bug/databus_mapping_regression/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "databus_mapping_regression"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.31.0"
+
+[dependencies]

test_programs/compile_success_no_bug/databus_mapping_regression/src/main.nr

@@ -0,0 +1,42 @@
+trait Empty {
+    fn empty() -> Self;
+}
+
+impl Empty for Field {
+    fn empty() -> Self {
+        0
+    }
+}
+
+pub fn is_empty<T>(item: T) -> bool where T: Empty + Eq {
+    item.eq(T::empty())
+}
+
+pub fn array_to_bounded_vec<T, let N: u32>(array: [T; N]) -> BoundedVec<T, N> where T: Empty + Eq {
+    let mut len = 0;
+    for elem in array {
+        if !is_empty(elem) {
+            len += 1;
+        }
+    }
+
+    BoundedVec { storage: array, len }
+}
+
+global TX_SIZE = 5;
+global APP_CALL_SIZE = 2;
+
+fn main(
+    a: call_data(0) [Field; TX_SIZE],
+    b: call_data(1) [Field; APP_CALL_SIZE]
+) -> return_data [Field; TX_SIZE] {
+    let mut a_as_bounded_vec = array_to_bounded_vec(a);
+
+    for i in 0..APP_CALL_SIZE {
+        let value = b[i];
+        if value != 0 {
+            a_as_bounded_vec.push(value);
+        }
+    }
+    a_as_bounded_vec.storage
+}