Reland D74436 "Change clang option -ffp-model=precise to select ffp-contract=on""

    Change clang option -ffp-model=precise, the default, to select ffp-contract=on
    The patch caused some problems for PowerPC but ibm has made
    adjustments so I am resubmitting this patch.  Additionally, Andy looked
    at the performance regressions on LNT and it looks like a loop
    unrolling decision that could be adjusted.

    Reviewers: rjmccall, Andy Kaylor

    Differential Revision: https://reviews.llvm.org/D74436