[SLP]Initialize the lane with the given value instead of default 0.

There is a bug in the reordering analysis stage. If the element with the
given hash is not added to the map but has the same number of APOs and
instructions with same parent, but different instruction opcode, it will
be initalized with default values and then the counter is increased by
1. But the lane is not updated and default to 0 instead of the actual
   `Lane` value. It leads to the fact that the analysis is useless in
   many cases and default to lane 0 instead of actual lane with the
   minimum amount of APO operands.

Differential Revision: https://reviews.llvm.org/D116690