[IR] Add a Instruction::dropPoisonGeneratingFlags helper

Summary:
The helper will be used in a later change.  This change itself is NFC
since the only user of this new function is its unit test.

Reviewers: majnemer, efriedma

Reviewed By: efriedma

Subscribers: efriedma, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D30184

llvm-svn: 296035