[IndVars] Drop "exact" flag from lshr and udiv when substituting their args

There is a transform that may replace `lshr (x+1), 1` with `lshr x, 1` in case
if it can prove that the result will be the same. However the initial instruction
might have an `exact` flag set, and it now should be dropped unless we prove
that it may hold. Incorrectly set `exact` attribute may then produce poison.

Differential Revision: https://reviews.llvm.org/D53061
Reviewed By: sanjoy

llvm-svn: 344223