AMDGPU: Try to unfold fneg source when matching legacy fmin/fmax

AMDGPU: Try to unfold fneg source when matching legacy fmin/fmax

This is NFC as it stands, since other combines will effectively
prevent this from being reachable. This will avoid regressions in a
future change which tries to make better use of select source
modifiers.

Didn't bother with the GlobalISel part for now, since the baseline
combine doesn't seem to work on the existing test.