[llvm] [SandboxVectorizer][NFCI] Fix use of possibly-uninitialized scalar. (PR #122201)

[via llvm-commits]

vporpo wrote:

@Sterling-Augustine shouldn't we always register the callback in the constructor, before the early return?

@tylanphear Setting it to max() may not be the correct fix, because `Ctx.unregisterEraseInstrCallback()` in the SeedCollector's constructor may not work well with it.

https://github.com/llvm/llvm-project/pull/122201