[SLP]Improve gathering of scalar elements.

1. Better sorting of scalars to be gathered. Trying to insert
   constants/arguments/instructions-out-of-loop at first and only then
   the instructions which are inside the loop. It improves hoisting of
   invariant insertelements instructions.
2. Better detection of shuffle candidates in gathering function.
3. The cost of insertelement for constants is 0.

Part of D57059.

Differential Revision: https://reviews.llvm.org/D103458