[SLP]Improve cost model for the vectorized extractelements.

1. No need to call `areAllUsersVectorized` as later the cost is
   calculated only if the instruction has one use and gets vectorized.
2. Need to calculate the cost of the dead extractelement more precisely,
   taking the vector type of the vector operand, not the resulting
   vector type.

Part of D57059.

Differential Revision: https://reviews.llvm.org/D99980