[SCEV][NFC][CT] Do not waste time proving contextual facts for unreached loops and blocks

In fact, in unreached code we can say that every fact is true. So do not waste time trying to
do something smarter.

Formally it's not an NFC because it may change query results in unreached code, but they
won't have any impact on execution.

Hypothetical CT boost expected but not measured in practice.

Differential Revision: https://reviews.llvm.org/D129878