[ty] Avoid merging snapshots if equivalent

[charliermarsh]

Summary

The idea here is: most of the time, most bindings don't change in an if block or equivalent. For example, given:

def f(x: int, y: str, z: list[int]) -> None:
    a = 1
    b = "hello"
    c = []
    if x > 0:
        a = 2

a changed, but all the other symbols were unchanged, so we can avoid running the full merge for the PlaceState for all other symbols.

This doesn't show up in any of our standard benchmarks, but it's a pretty simple optimization and shows a 38% speedup for an example like:

class BigClass:
    def __init__(self, data: dict[str, int]) -> None:
        if "field_0" in data: self.field_0 = data["field_0"]
        else: self.field_0 = 0
        ...  # 2000 fields

[astral-sh-bot]

Typing conformance results

No changes detected ✅

Current numbers

The percentage of diagnostics emitted that were expected errors held steady at 89.34%. The percentage of expected errors that received a diagnostic held steady at 85.30%. The number of fully passing files held steady at 87/134.

[astral-sh-bot]

Memory usage report

Memory usage unchanged ✅

[astral-sh-bot]

ecosystem-analyzer results

No diagnostic changes detected ✅

Full report with detailed diff (timing results)