[NFC][Driver] Clean up RenderFloatingPointOptions() (#91017)

This change refactors RenderFloatingPointOptions() to eliminate some
excessively complicated logic and a redundant switch statement. The
logic being simplified is an artifact of the original -ffp-model
implementation, and over time it has become unnecessary.

The handling of diagnostics related to the -ffp-contract option is
still a bit convoluted after this change. I will address that in a
subsequent patch because I think it will make sense to make some minor
changes to the driver behavior when that is cleaned up. The current
patch should not make any change to observable behavior of the driver.