fix: Ssa typing for array & slice indexes (#4278)

# Description

## Problem\*

Partial work towards #4275

## Summary\*

We were mixing Field and u64 types when accessing arrays. Since we index
arrays by u64s, always cast array/slice addressing values to u64.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** 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/ssa_gen/context.rs

@@ -746,6 +746,11 @@ impl<'a> FunctionContext<'a> {
         address
     }
 
+    /// Array indexes are u64s. This function casts values used as indexes to u64.
+    pub(super) fn make_array_index(&mut self, index: ValueId) -> ValueId {
+        self.builder.insert_cast(index, Type::unsigned(64))
+    }
+
     /// Define a local variable to be some Values that can later be retrieved
     /// by calling self.lookup(id)
     pub(super) fn define(&mut self, id: LocalId, value: Values) {

compiler/noirc_evaluator/src/ssa/ssa_gen/mod.rs

@@ -14,10 +14,7 @@ use noirc_frontend::{
 
 use crate::{
     errors::{InternalError, RuntimeError},
-    ssa::{
-        function_builder::data_bus::DataBusBuilder,
-        ir::{instruction::Intrinsic, types::NumericType},
-    },
+    ssa::{function_builder::data_bus::DataBusBuilder, ir::instruction::Intrinsic},
 };
 
 use self::{
@@ -390,8 +387,9 @@ impl<'a> FunctionContext<'a> {
         length: Option<super::ir::value::ValueId>,
     ) -> Result<Values, RuntimeError> {
         // base_index = index * type_size
+        let index = self.make_array_index(index);
         let type_size = Self::convert_type(element_type).size_of_type();
-        let type_size = self.builder.field_constant(type_size as u128);
+        let type_size = self.builder.numeric_constant(type_size as u128, Type::unsigned(64));
         let base_index =
             self.builder.set_location(location).insert_binary(index, BinaryOp::Mul, type_size);
 
@@ -428,20 +426,10 @@ impl<'a> FunctionContext<'a> {
         index: super::ir::value::ValueId,
         length: Option<super::ir::value::ValueId>,
     ) {
-        let array_len = length.expect("ICE: a length must be supplied for indexing slices");
-        // Check the type of the index value for valid comparisons
-        let array_len = match self.builder.type_of_value(index) {
-            Type::Numeric(numeric_type) => match numeric_type {
-                // If the index itself is an integer, keep the array length as a Field
-                NumericType::Unsigned { .. } | NumericType::Signed { .. } => array_len,
-                // If the index and the array length are both Fields we will not be able to perform a less than comparison on them.
-                // Thus, we cast the array length to a u64 before performing the less than comparison
-                NumericType::NativeField => self
-                    .builder
-                    .insert_cast(array_len, Type::Numeric(NumericType::Unsigned { bit_size: 64 })),
-            },
-            _ => unreachable!("ICE: array index must be a numeric type"),
-        };
+        let index = self.make_array_index(index);
+        // We convert the length as an array index type for comparison
+        let array_len = self
+            .make_array_index(length.expect("ICE: a length must be supplied for indexing slices"));
 
         let is_offset_out_of_bounds = self.builder.insert_binary(index, BinaryOp::Lt, array_len);
         let true_const = self.builder.numeric_constant(true, Type::bool());