CaptureTracking: Plug a loophole in the "too many uses" heuristic.

The heuristic was added to avoid spending too much compile time A specially
crafted test case (PR17461, PR16474) with many uses on a select or bitcast
instruction can still trigger the slow case. Add a check for that case.

This only affects compile time, don't have a good way to test it.

llvm-svn: 191896