[mypyc] feat: support negative index in TupleGet op

This PR modifies `TupleGet.__init__` to automatically convert negative indexes to positive indexes instead of crashing at the assert

This won't change functionality on its own, but will allow us to pass negative values in python#19972 so I think we should consider this PR a prerequisite to that one

Author: BobTheBuidler

mypyc/ir/ops.py

@@ -1045,10 +1045,15 @@ class TupleGet(RegisterOp):
 
     def __init__(self, src: Value, index: int, line: int = -1, *, borrow: bool = False) -> None:
         super().__init__(line)
+        assert isinstance(
+            src.type, RTuple
+        ), "TupleGet only operates on tuples, not {type(src.type).__name__}"
+        src_len = len(src.type.types)
         self.src = src
         self.index = index
-        assert isinstance(src.type, RTuple), "TupleGet only operates on tuples"
-        assert index >= 0
+        if index < 0:
+            self.index += src_len
+        assert self.index <= src_len - 1, f"Index out of range.\nsource type: {src.type}\nindex: {index}"
         self.type = src.type.types[index]
         self.is_borrowed = borrow
 
@@ -1073,11 +1078,19 @@ class Cast(RegisterOp):
 
     error_kind = ERR_MAGIC
 
-    def __init__(self, src: Value, typ: RType, line: int, *, borrow: bool = False) -> None:
+    def __init__(
+        self, src: Value, typ: RType, line: int, *, borrow: bool = False, unchecked: bool = False
+    ) -> None:
         super().__init__(line)
         self.src = src
         self.type = typ
+        # If true, don't incref the result.
         self.is_borrowed = borrow
+        # If true, don't perform a runtime type check (only changes the static type of
+        # the operand). Used when we know that the cast will always succeed.
+        self.is_unchecked = unchecked
+        if unchecked:
+            self.error_kind = ERR_NEVER
 
     def sources(self) -> list[Value]:
         return [self.src]
@@ -1213,6 +1226,7 @@ def __init__(
         var_arg_idx: int = -1,
         *,
         is_pure: bool = False,
+        returns_null: bool = False,
     ) -> None:
         self.error_kind = error_kind
         super().__init__(line)
@@ -1227,7 +1241,10 @@ def __init__(
         # and all the arguments are immutable. Pure functions support
         # additional optimizations. Pure functions never fail.
         self.is_pure = is_pure
-        if is_pure:
+        # The function might return a null value that does not indicate
+        # an error.
+        self.returns_null = returns_null
+        if is_pure or returns_null:
             assert error_kind == ERR_NEVER
 
     def sources(self) -> list[Value]: