JIT: speed up fgComputePreds (#35352)

JIT: speed up fgComputePreds (#35352)

Interaction of `fgComputePreds` and `fgAddRefPred` could be quadratic in the
number of preds.

Usually the number of preds is small (1 or 2) but in some cases seen from
compiled regular expressions it could be in the thousands. On one such case
a single call to fgComputePreds was taking ~20% of jit time.

Since we build the pred list in sorted order we can take advantage of this
to avoid searching the list for potential duplicates in `fgAddRefPred` when
it is called from `fgComputePreds` -- the only possible duplicate entry is
at the end of the list.

This doesn't address perf of subsequent calls to `fgAddRefPred` but likely
those happen somewhat randomly and are unlikely to be as costly.