[SLP]Fix the crash on cost calculation if non-compatible vectors shuffled.

If the extracts from the non-power-2 vectors are recognized as shuffles,
need some extra checks to not crash cost calculations if trying to gext
the ecost for subvector extracts. In this case need to check carefully
that we do not exit out of bounds of the original vector, otherwise the
TTI's cost model will crash on assert.

Differential Revision: https://reviews.llvm.org/D101477