[SLP]Improve calculations of the cost for reused/reordered scalars.

Part of D105020. Also, fixed FIXMEs that need to use wider vector type
when trying to calculate the cost of reused scalars. This may cause
regressions unless D100486 is landed to improve the cost estimations
for long vectors shuffling.

Differential Revision: https://reviews.llvm.org/D106060