[LoopVectorize] propagate fast-math-flags from induction instructions

This code assumed that FP math was only permissable if it was
fully "fast", so it hard-coded "fast" when creating new instructions.

The underlying code already allows matching recurrences/reductions
that are only "reassoc", so this change should prevent the potential
miscompile seen in the test diffs (we created "fast" ops even though
none existed in the original code).

I don't know if we need to create the temporary IRBuilder objects
used here, so that could be follow-up clean-up.

There's an open question about whether we should require "nsz" in
addition to "reassoc" here. InstCombine uses that combo for its
reassociative folds, but I think codegen is not as strict.