Unnecessary cursor restores

[ojwb]

I noticed that we're generating unnecessary cursor restores for or in some cases.

The simplest example is probably:

'a' or 'b'

K_needed() reports that this can change the cursor - that's true (because 'a' moves the cursor if it matches) but for generating the or itself what actually matters is if a clause can move the cursor and fail. That's still possible, e.g. we do need to restore the cursor if the first clause fails here:

('a' 'a') or 'b'

However there are a lot of common cases that can't change the cursor on failure - e.g. an or clause which is a single literal string or grouping.

We should also be checking this per-clause (currently we restore after every clause or after none).

[ojwb]

Related is that we only need to restore the cursor between items in an or, not after the final item, so this example never needs to restore the cursor, even if some_routine might modify the cursor and fail:

'some_string' or some_grouping or some_routine

(The cursor will get restored if some_routine fails, assuming the failure is handled by something rather than terminating the Snowball program - but the code generated for the or doesn't need to do it.)