[CostModel] Make target intrinsics cheap by default

This patch changes the intrinsics cost model to assume that by default
target intrinsics are cheap. This didn't seem to be the case for all
intrinsics, and is potentially an MVE problem due to our scalarization
overheads. Cheap seems to be a good default in general though.

Differential Revision: https://reviews.llvm.org/D90597