[JuliaLowering] Equalize ssavalues in `SyntaxTree` and `CodeInfo` (#61673)

Right now, we extract `(meta nospecialize)` from our own slot objects in
`to_lowered_expr`, but that means the number of IR statements in the
CodeInfo
doesn't match the SyntaxTree IR coming out of lowering. This makes
generation
of DebugInfo messier: we don't know the IR structure before
`to_lowered_expr`
and we don't have any provenance left after it, so we must interleave
debuginfo
creation with `to_lowered_expr` (and generally maintain state during
that
function, remapping ssavalues, etc).

This change just aligns the AST part of `st5` with `CodeInfo` so
something like
`make_debuginfo(st5::SyntaxTree)` is possible.

Co-authored-by: Cody Tapscott <84105208+topolarity@users.noreply.github.com>