Driver: extract floating point optimization handling (NFC)

Extract the logic for the floating point handling into its own function.
None of this information is needed for calculating the remainder of the
arguments to the frontend.  NFC

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312385 91177308-0d34-0410-b5e6-96231b3b80d8