[SLP]Improve vectorization of split loads.

Need to fix ther cost estimation for split loads, since we look at the
subregs already, no need to permute them, need just to estimate
subregister insert, if it is smaller than the real register. Also, using
split loads, it might be profitable already to vectorize smaller trees
with gathering of the loads.

Differential Revision: https://reviews.llvm.org/D107188