[SCEV] Do not cache S -> V if S is not equivalent of V

[SCEV] Do not cache S -> V if S is not equivalent of V

SCEV tracks the correspondence of created SCEV to original instruction.
However during creation of SCEV it is possible that nuw/nsw/exact flags are
lost.

As a result during expansion of the SCEV the instruction with nuw/nsw/exact
will be used where it was expected and we produce poison incorreclty.

Reviewers: sanjoy, mkazantsev, sebpop, jbhateja
Reviewed By: sanjoy
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D41578

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322058 91177308-0d34-0410-b5e6-96231b3b80d8