Type-assert vconverts to circumvent inference restriction on recursion

[GianlucaFuwa]

Fixes JuliaSIMD/LoopVectorization.jl#526, building upon @chriselrod's last commits a month or two ago, by type-asserting the return types of vconvert in the relevant cases.

Digging around with Cthuhlu's @descend I found that the issue, at least when I tested on 1.10.2, originates from the recursive calling of _vconvert in promote which caused the compiler to give up on inference (I assume this became more restrictive in 1.10).
Not sure if this solution is "legal" in the sense that vconvert always returns the type I annotated.

If this is fine, I would also add tests in LV's "test/" directory.