Bugfix: Moves AOT snapshot input verification past where the last input
is added to the inputs list.
Cleanup:
* Extracts _isValidAotPlatform method.
* Moves non-platform-specific logic to the top.
* Moves variable declaration closer to first use, and inlines to a
narrower scope where possible.
This relands #17136, which was reverted in #17142 due to breakage in
on-device iOS debug builds.
c3d43ef4a7