[llvm][misexpect] Avoid division by 0 when using sample profiling

MisExpect diagnostics should not prevent compilation from succeeding, and the
assertion is insufficient to prevent division by zero in release builds.

This patch addresses that by replacing the assert with an early return.

Additionally, it disables MisExpect diagnostics when using sample profiling,
since this is the only known case where this error has manifested.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D124302