Clean up the checks in the fast-math driver test

This change replaces most of the cc1 option checks in the driver test for fast-math option handling. These changes rely on the assumption that the order in which the driver emits floating-point options is stable.
  
The changes also rely on the assumption that the order of prefixes listed on the FileCheck command line is unimportant and that all prefixed checks will be combined and checked as if they were a single prefix. At the time of the change, that worked.