[NFCI][CostModel] Refactor getIntrinsicInstrCost

Combine the two API calls into one by introducing a structure to hold
the relevant data. This has the added benefit of moving the boiler
plate code for arguments and flags, into the constructors. This is
intended to be a non-functional change, but the complicated web of
logic involved here makes it very hard to guarantee.

Differential Revision: https://reviews.llvm.org/D79941