[CostModel] split handling of intrinsics from other calls

This should be close to NFC (no-functional-change), but I
can't completely rule out that some call on some target
travels down a different path. There's an especially large
amount of code spaghetti in this part of the cost model.

The goal is to clean up the intrinsic cost handling so
we can canonicalize to the new min/max intrinsics without
causing regressions.