[LV] Move exact FP math check out of Requirements.

We know if the loop contains FP instructions preventing vectorization
after we are done with legality checks. This patch updates the code the
check for un-vectorizable FP operations earlier, to avoid unnecessarily
running the cost model and picking a vectorization factor. It also makes
the code more direct and moves the check to a position where similar
checks are done.

I might be missing something, but I don't see any reason to handle this
check differently to other, similar checks.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D98633