fix #26743, spurious return path in try-finally in tail position

[JeffBezanson]

This works by using a special value (#f) for expressions that don't yield values (return, goto), and then avoiding performing assignments and returns of those pseudo-values.

Example:

f(x) = (y = return x)

Before:

julia> @code_lowered f(0)
CodeInfo(:(begin 
        nothing
        return x
        y = nothing
        return
    end))

After:

julia> @code_lowered f(0)
CodeInfo(:(begin
      nothing
      Core.NewvarNode(:(y))
      return x
  end))