Follow up to 9fd9d56d, avoid a memory leak

Gaps in the basic block number range (from blocks being deleted or folded)
get block-value-tables allocated but never ejected, leading to a memory
leak, currently tripping up the asan buildbots. Fix this up by manually
freeing that memory.

As suggested elsewhere, if these things were owned by a unique_ptr then
cleanup would happen automagically. D118774 should eliminate the need for
this dance.