[mypyc] feat: support constant folding in translate_index_expr [1/1] (#19972)

This PR attempts to constant fold the index value in
`translate_index_expr`

I'm not sure any test changes are warranted for a small PR of this
nature.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: BobTheBuidler

mypyc/irbuild/expression.py

@@ -590,8 +590,12 @@ def transform_index_expr(builder: IRBuilder, expr: IndexExpr) -> Value:
 
     base = builder.accept(expr.base, can_borrow=can_borrow_base)
 
-    if isinstance(base.type, RTuple) and isinstance(index, IntExpr):
-        return builder.add(TupleGet(base, index.value, expr.line))
+    if isinstance(base.type, RTuple):
+        folded_index = constant_fold_expr(builder, index)
+        if isinstance(folded_index, int):
+            length = len(base.type.types)
+            if -length <= folded_index <= length - 1:
+                return builder.add(TupleGet(base, folded_index, expr.line))
 
     if isinstance(index, SliceExpr):
         value = try_gen_slice_op(builder, base, index)