Only verify LazyCallGraph under expensive checks

These verify calls are causing a lot of slowdown on some files, up to 8x.
The LazyCallGraph infra has been tested a lot over the years, so I'm fairly confident that we don't always need to run the verifys.

These verifies took >90% of total time in one of the compilations I looked at.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D97225