stage2: fix non-power-of-2-bits vector pointer cases

[kkHAIKE]

fix: https://github.com/ziglang/zig/issues/12812

some message: https://github.com/ziglang/zig/issues/12812#issuecomment-1256006923

runtime-known index case is too hard for me now, so simple add TODO and assert for it. 😊

[andrewrk]

Next step: rebase against master branch and get the CI tests passing.

[kkHAIKE]

sorry i added a wrong patch earlier..

i think may be add a func like packedStructToInt/intToPackedStruct for vector

[andrewrk]

Alternate solution landed in 8a0a6b7387fcd0017db85de14793abfd6ec7f6e5.