perf(core): loop through `oldCh` from `start + 1`

[Fyerl]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [x] Other, please describe: improve vdom diffing

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• [ ] New/updated tests are included

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

function findIdxInOld (node, oldCh, start, end) {
  for (let i = start + 1; i < end; i++) {
    const c = oldCh[i]
    if (isDef(c) && sameVnode(node, c)) return i
  }
}

loop through oldCh from start + 1, cause oldCh[start] is same with oldStartVnode and this comparison has already been done in the L444