[IndVars] Strengthen restricton in rewriteLoopExitValues

For some unclear reason rewriteLoopExitValues considers recalculation
after the loop profitable if it has some "soft uses" outside the loop (i.e. any
use other than call and return), even if we have proved that it has a user inside
the loop which we think will not be optimized away.

There is no existing unit test that would explain this. This patch provides an
example when rematerialisation of exit value is not profitable but it passes
this check due to presence of a "soft use" outside the loop.

It makes no sense to recalculate value on exit if we are going to compute it
due to some irremovable within the loop. This patch disallows applying this
transform in the described situation.

Differential Revision: https://reviews.llvm.org/D51581
Reviewed By: etherzhhb

llvm-svn: 345708