[TTI] Remove getOperationCost

This API call has been used recently with, a very valid, expectation
that it would do something useful but it doesn't actually query any
backend information. So, remove this method and merge its
functionality into getUserCost. As well as that, also use
getCastInstrCost to get a proper cost from the backend for the
concerned instructions though we only currently return the answer if
it's considered free. The default implementation now also checks
int/ptr conversions too, as well as truncs and bitcasts.

Differential Revision: https://reviews.llvm.org/D76124