[LoopVectorize] Check the number of uses of an FAdd before classifying as ordered

checkOrderedReductions looks for Phi nodes which can be classified as in-order,
meaning they can be vectorised without unsafe math. In order to vectorise the
reduction it should also be classified as in-loop by getReductionOpChain, which
checks that the reduction has two uses.

In this patch, a similar check is added to checkOrderedReductions so that we
now return false if there are more than two uses of the FAdd instruction.
This fixes PR52515.

Reviewed By: fhahn, david-arm

Differential Revision: https://reviews.llvm.org/D114002