[TableGen] Allow empty terminator in SequenceToOffsetTable (#119751)
Some clients do not want to emit a terminator after each sub-sequence
(they have other means of determining the length of sub-sequences).
This moves `Term` argument from `emit` method to the constructor and
makes it optional. It couldn't be made optional while still on the
`emit` method because if the terminator wasn't specified, it has to be
taken into account in `layout` method as well.
The fact that `layout` method was called is now recorded in a dedicated
member variable, `IsLaidOut`. `Entries != 0` can no longer be used to
reliably check if `layout` method was called because it may be zero for
a different reason: the terminator wasn't specified and all added
sequences (if any) were empty.
This reduces the size of `*LaneMaskLists` and `*SubRegIdxLists` a bit
and resolves the removed TODO.
llvm-project
c9070cce - [TableGen] Allow empty terminator in SequenceToOffsetTable (#119751)
