FIXED: JIT clause indexing could mis the right argument in the following
scenario:

  - There is a best index Ib, which is poor.
  - There is a normally better index I2, but the corresponding
    argument is not indexable.

Although the scenario is not that common, it can lead to a huge
slowdown.  Found by Jos de Roo.
reverted
Merge remote-tracking branch 'upstream/master'