fastmath: Fix `--math-mode=ieee` handling for `muladd_float` (#61708)

Even without `reassoc` and with identical optimization flags, `contract`
alone is enough to see divergent floating-point results from identical
code so IEEE mode must not set it.

This change means that `muladd` never fuses under `--math-mode=ieee`
which is consistent with the existing semantics (fuse when possible, but
no guarantees)