[SCEV] Reduce the number of invocation to non trivial getExact function

The invocation of getExact in ScalarEvolution::getBackedgeTakenInfo is used
only for getting statistic and for assert. 
Even if statistics is disabled, the code related to it will be eliminated
the invocation to getExact itself will not be eliminated
because it may have side-effects like creation of new SCEVs.

So do invocation only when we collect statistics or executes asserts.

Reviewers: mkazantsev, sanjoy, javed.absar
Reviewed By: javed.absar
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D46178

llvm-svn: 331099