Add LIKELY/UNLIKELY annotations to some if statements

A microoptimization we can apply much later in the future, especially for error checking where we don't care about the latency of failing the check.

The issue names no files, tests, or entry points; begin by locating the error-checking if statements in the repository and checking how compiler branch annotations are currently supported. Confirm which checks are suitable and define a benchmark or test showing that the annotations are correct before considering the work done.

Written by the indexing model from the issue text.