Relax constraints for reduction vectorization

Relax constraints for reduction vectorization

Summary:
Gating vectorizing reductions on *all* fastmath flags seems unnecessary;
`reassoc` should be sufficient.

Reviewers: tvvikram, mkuper, kristof.beyls, sdesmalen, Ayal

Reviewed By: sdesmalen

Subscribers: dcaballe, huntergr, jmolloy, mcrosier, jlebar, bixia, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57728

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355868 91177308-0d34-0410-b5e6-96231b3b80d8