[ScalarEvolutionExpander] Reuse findExistingExpansion during expansion cost...

[ScalarEvolutionExpander] Reuse findExistingExpansion during expansion cost calculation for division

Primary purpose of this change is to reuse existing code inside findExistingExpansion. However it introduces very slight semantic change - findExistingExpansion now looks into exiting blocks instead of a loop latches. Originally heuristic was based on the fact that we want to look at the loop exit conditions. And since all exiting latches will be listed in the ExitingBlocks, heuristic stays roughly the same.

Differential Revision: http://reviews.llvm.org/D12008

llvm-svn: 245227