[AMDGPU] Update isFMAFasterThanFMulAndFAdd assumptions

[critson]

Summary

MAD/MAC is no longer always being available.

[critson]

Technically this is NFC with current hardware configurations, but I would like to be able to decouple this from MAD/MAC.

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

 // a fast fma device and require denormals.
 //
 bool SITargetLowering::isFMAFasterThanFMulAndFAdd(const MachineFunction &MF,
                                                   EVT VT) const {
   VT = VT.getScalarType();
 
   switch (VT.getSimpleVT().SimpleTy) {
   case MVT::f32: {
-    // This is as fast on some subtargets. However, we always have full rate f32
-    // mad available which returns the same result as the separate operations
-    // which we should prefer over fma. We can't use this if we want to support
-    // denormals, so only report this in these cases.
+    // If mad is not available this depends only on if f32 fma is full rate.
+    if (!Subtarget->hasMadMacF32Insts())
+      return Subtarget->hasFastFMAF32();
+
+    // Otherwise f32 mad is always full rate and returns the same result as
+    // the separate operations so should be preferred over fma.
+    // However does not support denomals.
     if (hasFP32Denormals(MF))
       return Subtarget->hasFastFMAF32() || Subtarget->hasDLInsts();
 
     // If the subtarget has v_fmac_f32, that's just as good as v_mac_f32.
     return Subtarget->hasFastFMAF32() && Subtarget->hasDLInsts();
   }
   case MVT::f64:
     return true;