When I added https://bugs.webkit.org/show_bug.cgi?id=197265 I assumed that which block is the root does not change in the middle of strength reduction.
But specializeSelect can use splitForward, which for some odd reason uses a new block for the first half of the given block (instead of using a new block for the second half, as I expected).
So if the block being split is the root block I must update m_root and erase the m_valueInConstant cache.
Erasing the cache cannot cause wrong results: at most it can make us miss some optimization opportunities in this iteration of the fixpoint.
*** This bug has been marked as a duplicate of bug 197756 ***