feat: skip reading values immediately after it being written into an …

…array (#5449)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

I noticed a couple of cases where we were writing into an array and then
immediately reading the same index from it. This seems like something
that we should be able to unwind.

Draft as I want to do more thinking on how `EnableSideEffects` can
interact with this.

## 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\*

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

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

@@ -602,6 +602,18 @@ impl Instruction {
                 }
             }
             Instruction::ArrayGet { array, index } => {
+                if let Value::Instruction { instruction, .. } = &dfg[*array] {
+                    if let Instruction::ArraySet { index: write_index, value, .. } =
+                        dfg[*instruction]
+                    {
+                        // If we're reading from an index of the array which we just wrote to, we can return
+                        // the value which we wrote without performing the read.
+                        if dfg.resolve(write_index) == dfg.resolve(*index) {
+                            return SimplifiedTo(value);
+                        }
+                    }
+                }
+
                 let array = dfg.get_array_constant(*array);
                 let index = dfg.get_numeric_constant(*index);
                 if let (Some((array, _)), Some(index)) = (array, index) {