[SCEV] Only verify BECounts for reachable loops (PR50523)

For unreachable loops, any BECount is legal, and since D98706 SCEV
can make use of this for loops that are unreachable due to constant
branches. To avoid false positives, adjust SCEV verification to only
check BECounts in reachable loops.

Fixes https://github.com/llvm/llvm-project/issues/50523.

Differential Revision: https://reviews.llvm.org/D120651