[TabelGen] Use ID{n-m} for outer let statements (#187436)
I found this occasionally.
For outer let statements, if we want to override some bits, we specify
the range list in the form of `<n-m>`. But for inner let statements,
we use `{n-m}`.
This is inconsistent, and I can't find the reason why it is designed
as this. So here we make inner/outer let statements consistent and
remove the duplicated parsing functions.
There is only one in-tree usage so I think the impact is small.
