chore(brillig): No temp reg for size when generating heap vector (#11813)

Author: vezenovm

compiler/noirc_evaluator/src/brillig/brillig_ir/codegen_memory.rs

@@ -285,9 +285,7 @@ impl<F: AcirField + DebugToString, Registers: RegisterAllocator> BrilligContext<
     ) -> Allocated<HeapVector, Registers> {
         let heap_vector = self.allocate_heap_vector();
 
-        // Read the size using the dedicated helper function
-        let size_variable = self.codegen_read_vector_size(vector);
-        self.mov_instruction(heap_vector.size, size_variable.address);
+        self.codegen_read_vector_size_into(vector, heap_vector.size);
 
         // Get the pointer to the items using the dedicated helper function
         self.codegen_vector_items_pointer(vector, heap_vector.pointer);
@@ -334,19 +332,28 @@ impl<F: AcirField + DebugToString, Registers: RegisterAllocator> BrilligContext<
         self.codegen_read_rc(vector.pointer)
     }
 
-    /// Returns a variable holding the size of a given vector.
-    pub(crate) fn codegen_read_vector_size(
+    /// Reads the size of a given vector into `dest`.
+    pub(crate) fn codegen_read_vector_size_into(
         &mut self,
         vector: BrilligVector,
-    ) -> Allocated<SingleAddrVariable, Registers> {
-        let result = self.allocate_single_addr_usize();
+        dest: MemoryAddress,
+    ) {
         self.codegen_usize_op(
             vector.pointer,
-            result.address,
+            dest,
             BrilligBinaryOp::Add,
             assert_usize(offsets::VECTOR_SIZE),
         );
-        self.load_instruction(result.address, result.address);
+        self.load_instruction(dest, dest);
+    }
+
+    /// Returns a variable holding the size of a given vector.
+    pub(crate) fn codegen_read_vector_size(
+        &mut self,
+        vector: BrilligVector,
+    ) -> Allocated<SingleAddrVariable, Registers> {
+        let result = self.allocate_single_addr_usize();
+        self.codegen_read_vector_size_into(vector, result.address);
         result
     }