* [TVMScript][Fix] Correct round-trip of explicit root block

Prior to this commit, when converting TIR to TVMScript, the root
`tir::Block` is typically hidden.  When parsing, however,
`tvm::tir::ScriptComplete` will wrap the function body in a root block
if the primfunc if the contains at least one block and does not
already have a root block.  As a result, if the root block is the only
block present, it would be stripped by a round-trip.

This commit tightens the condition for hiding the root `tir::Block`
when converting to TVMScript, so that it is printed in cases where
the autocompleter would reinsert it when parsing.