treat when(index() > -1) as a boolean expression

RT #133368

when(X) is normally compiled as when($_ ~~ X) *except* when X appears to
be a boolean expression, in which case it's used directly.

5.28.0 introduced an optimisation whereby comparisons involving index
like

    index(...) != -1

eliminated the comparison, and pp_index() returned a boolean value
directly. This defeated the 'look for a boolean op' mechanism, and so

    when(index(...) != -1)

and similar were being incorrectly compiled as

    when($_ ~~ (index(...) != -1))

0 files changed, 0 insertions, 0 deletions