[SLP]Improve multinode analysis.

Changes the preliminary multinode analysis:
1. Introduced scores for reversed loads/extractelements.
2. Improved shallow score calculation.
3. Lowered the cost of external uses (no need to consider it several times, just ones).
4. The initial lane for analysis is the one with the minimal possible
   reorderings.

These changes in general shall reduce compile time and improve the
reordering in many cases.

Part of D57059.

Differential Revision: https://reviews.llvm.org/D101109