[Reassociate] Make sure EraseInst sets MadeChange

Summary:
EraseInst didn't report that it made IR changes through MadeChange.

It is essential that changes to the IR are reported correctly,
since for example ReassociatePass::run() will indicate that all
analyses are preserved otherwise.
And the CGPassManager determines if the CallGraph is up-to-date
based on status from InstructionCombiningPass::runOnFunction().

Reviewers: craig.topper, rnk, davide

Reviewed By: rnk, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34616

llvm-svn: 306368