[vectorizer] Simplify code to use existing helpers on the Function
object and fewer pointless variables.

Also, add a clarifying comment and a FIXME because the code which
disables *all* vectorization if we can't use implicit floating point
instructions just makes no sense at all.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200214 91177308-0d34-0410-b5e6-96231b3b80d8