Speed up make_simplified_union (#14370)

If there is only one non-union item, there's nothing interesting to do.
This is pretty common, and it avoids a fairly expensive
`_remove_redundant_union_items` call.

(Various small optimizations, including this, together netted a 6%
performance improvement in self check.)

Author: JukkaL

mypy/typeops.py

@@ -441,6 +441,7 @@ def make_simplified_union(
     * [int, int] -> int
     * [int, Any] -> Union[int, Any] (Any types are not simplified away!)
     * [Any, Any] -> Any
+    * [int, Union[bytes, str]] -> Union[int, bytes, str]
 
     Note: This must NOT be used during semantic analysis, since TypeInfos may not
           be fully initialized.
@@ -455,10 +456,14 @@ def make_simplified_union(
     # Step 1: expand all nested unions
     items = flatten_nested_unions(items)
 
-    # Step 2: remove redundant unions
+    # Step 2: fast path for single item
+    if len(items) == 1:
+        return get_proper_type(items[0])
+
+    # Step 3: remove redundant unions
     simplified_set: Sequence[Type] = _remove_redundant_union_items(items, keep_erased)
 
-    # Step 3: If more than one literal exists in the union, try to simplify
+    # Step 4: If more than one literal exists in the union, try to simplify
     if (
         contract_literals
         and sum(isinstance(get_proper_type(item), LiteralType) for item in simplified_set) > 1
@@ -471,7 +476,7 @@ def make_simplified_union(
     if nitems > 1 and (
         nitems > 2 or not (type(items[0]) is NoneType or type(items[1]) is NoneType)
     ):
-        # Step 4: At last, we erase any (inconsistent) extra attributes on instances.
+        # Step 5: At last, we erase any (inconsistent) extra attributes on instances.
 
         # Initialize with None instead of an empty set as a micro-optimization. The set
         # is needed very rarely, so we try to avoid constructing it.