[x86] add tests for fmax/fmin experimental intrinsics with 'fast' FMF; NFC

D87391 proposes to change the lowerings for 'nnan'-only FMF.
That's the minimal requirement to get good codegen for x86,
but currently we have bugs hindering that output unless the
full 'fast' FMF is applied. These tests provide coverage for
the ideal lowerings.