[ty] Eagerly simplify 'True' and 'False' constraints (#18998)

## Summary

Simplifies literal `True` and `False` conditions to `ALWAYS_TRUE` /
`ALWAYS_FALSE` during semantic index building. This allows us to eagerly
evaluate more constraints, which should help with performance (looks
like there is a tiny 1% improvement in instrumented benchmarks), but
also allows us to eliminate definitely-unreachable branches in
control-flow merging. This can lead to better type inference in some
cases because it allows us to retain narrowing constraints without
solving https://github.com/astral-sh/ty/issues/690 first:
```py
def _(c: int | None):
    if c is None:
        assert False
    
    reveal_type(c)  # int, previously: int | None
```

closes https://github.com/astral-sh/ty/issues/713

## Test Plan

* Regression test for https://github.com/astral-sh/ty/issues/713
* Made sure that all ecosystem diffs trace back to removed false
positives

crates/ty_python_semantic/resources/mdtest/narrow/while.md

@@ -59,3 +59,17 @@ while x != 1:
 
     x = next_item()
 ```
+
+## With `break` statements
+
+```py
+def next_item() -> int | None:
+    return 1
+
+while True:
+    x = next_item()
+    if x is not None:
+        break
+
+reveal_type(x)  # revealed: int
+```