[Attributor] Use assumed information to determine side-effects

We relied on wouldInstructionBeTriviallyDead before but that functions
does not take assumed information, especially for calls, into account.
The replacement, AAIsDead::isAssumeSideEffectFree, does.

This change makes AAIsDeadCallSiteReturn more complex as we can have
a dead call or only dead users.

The test have been modified to include a side effect where there was
none in order to keep the coverage.

Reviewed By: sstefan1

Differential Revision: https://reviews.llvm.org/D73311