[DAGCombiner] Check term use before applying aggressive FSUB optimisations

Summary:
Without this check unnecessary FMA instructions are generated when the FSUB terms are reused.
This also has the side-effect that the same value is computed to different levels of precision, which can create undesirable effects if the results are used together in subsequent computation.

Reviewers: arsenm, nhaehnle, foad, tpr, dstuttard, spatel

Reviewed By: arsenm

Subscribers: jvesely, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71656