trunk/4511f207fd60b6160d0b434ad0e8237901348e79: Remove stale "loop is faster than ={0}" workaround in vec (#184889)

The 2020-era GCC quirk (#32502) the loops worked around no longer applies. On PyTorch's minimum compilers (GCC 11.3, Clang 16, MSVC VS 16.10), the loop and = {} brace-init emit identical assembly (verified via godbolt).

Zero-init sites use = {}.

Test Plan: existing CI; no runtime behavior change.

Authored by Claude.

Pull Request resolved: #184889

Approved by: https://github.com/jansel