[LoopVectorize] Improve getVectorCallCost() getScalarizationOverhead() call.

By calling getScalarizationOverhead with the CallInst instead of the types of
its arguments, we make sure that only unique call arguments are added to the
scalarization cost.

getScalarizationOverhead() is extended to handle calls by only passing on the
actual call arguments (which is not all the operands).

This also eliminates a wrapper function with the same name.

review: Hal Finkel
llvm-svn: 293459