[LoopVectorize] Fix zext/sext cost calculations when types are shrunk

In getInstructionCost if we know a zext/sext is going to be shrunk
we should only be changing the destination type, and leave the
source type unchanged. For example, we may change a zext from

  zext <16 x i8> %a to <16 x i32>

to

  zext <16 x i8> %a to <16 x i16>

However, we were previously calculating the cost for doing

  zext <16 x i16> %a to <16 x i16>

which is incorrect.

Differential Revision: https://reviews.llvm.org/D147152