lowering: keep always-defined flag on slots more often (#61720)

Easier parts extracted from
https://github.com/JuliaLang/julia/pull/61387. Since a vinfo bit was
reused many years ago, we currently erase always-defined status when
deciding to unbox locals, so they're outlined in the IR where they don't
need to be, as seen in some MTK-generated code.

This change also puts non-captured, single-assign variables through
`lambda-optimize-vars` so they get a chance to gain the always-defined
flag.