[TargetTransformInfo, API] Add a list of operands to TTI::getUserCost

The changes are a result of discussion of https://reviews.llvm.org/D33685.
It solves the following problem:

1. We can inform getGEPCost about simplified indices to help it with
   calculating the cost. But getGEPCost does not take into account the
   context which GEPs are used in.
2. We have getUserCost which can take the context into account but we cannot
   inform about simplified indices.

With the changes getUserCost will have access to additional information
as getGEPCost has.

The one parameter getUserCost is also provided.

Differential Revision: https://reviews.llvm.org/D34057

llvm-svn: 306674