Fix representation of SORT_TYPE_STILL_IN_PROGRESS.

It turns out that the code did indeed rely on a zeroed
TuplesortInstrumentation.sortMethod field to indicate
"this worker never did anything", although it seems the
issue only comes up during certain race-condition-y cases.

Hence, rearrange the TuplesortMethod enum to restore
SORT_TYPE_STILL_IN_PROGRESS to having the value zero,
and add some comments reinforcing that that isn't optional.

Also future-proof a loop over the possible values of the enum.
sizeof(bits32) happened to be the correct limit value,
but only by purest coincidence.

Per buildfarm and local investigation.

Discussion: https://postgr.es/m/[email protected]

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