[StandardInstrumentations] Only turn on -verify-cfg-preserved by default in expensive checks builds

https://llvm-compile-time-tracker.com/compare.php?from=3fd42f50d8aadb4d0c348ac17cd2115c1b0564a4&to=50c37f6fc62a1e7bb4f0e307c89f760d42dbe4e9&stat=instructions:u
shows that this is fairly expensive, 5-10% increase in compile time, and I'd like to add more similar checks under the same flag.

This matches the legacy pass manager.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D146068