De-pessimize ScalarEvolution's handling of nsw/nuw?

[Sanjoy Das]

> The comments in the source code say that ScalarEvolution does not
> apply an instruction's nsw/nuw flags to the corresponding SCEV
> expression because another non-control-equivalent instruction without
> nsw/nuw can be mapped to the same expression.

I think the problem is that another non-control-equivalent instruction
*with* nsw/nuw can be mapped to the same expression and you may end up
introducing nsw / nuw operations in paths that didn't have any to
begin with.  IOW since you "forget" the control dependence when
translating a value to a scev, you cannot rematerialize or reason
about the scev in arbitrary points in your program if the scev has
potential to introduce UB.

Having said that, I don't fully understand poison and LLVM's
overflowing semantics; and it may be that it is possible to justify
mapping nsw binary ops to nsw scevs under certain conditions or
maybe even always.  But it is not something that is obviously true.

-- Sanjoy