[IndVars] Use symbolic max block exit count to handle the last iter

Old logic: when loop symbolic max exit count matched *exact* block exit count,
assume that all subsequent blocks will do 1 iteration less.

New logic: when loop symbolic max exit count matched *symbolic max* block exit count,
assume that all subsequent blocks will do 1 iteration less.

The new logic is still legal and is more permissive in situations when exact
block exit count is not known.

Differential Revision: https://reviews.llvm.org/D139692
Reviewed By: nikic