AMDGPU: Avoid instantiating PatFrag with null_frag (llvm#77271)

This makes it possible to pass null_frag to the MAIInst multiclass.

null_frag does not work as you may hope if used as the input to a
PatFrag, which is what happens when it's passed through to *MAIFrag.
Avoid this by checking for null_frag. It might be possible to hack up
tablegen to allow consuming PatFrag inputs.

Author: arsenm

llvm/lib/Target/AMDGPU/VOP3PInstructions.td

@@ -623,25 +623,26 @@ multiclass MAIInst<string OpName, string P, SDPatternOperator node,
     // FP32 denorm mode is respected, rounding mode is not. Exceptions are not supported.
     let Constraints = !if(NoDstOverlap, "@earlyclobber $vdst", "") in {
       def _e64 : MAIInst<OpName, !cast<VOPProfileMAI>("VOPProfileMAI_" # P),
-                         !if(NoDstOverlap, null_frag, AgprMAIFrag<node>)>,
+                         !if(!or(NoDstOverlap, !eq(node, null_frag)), null_frag, AgprMAIFrag<node>)>,
                  MFMATable<0, NAME # "_e64">;
 
       let SubtargetPredicate = isGFX90APlus, Mnemonic = OpName in
       def _vgprcd_e64 : MAIInst<OpName # "_vgprcd", !cast<VOPProfileMAI>("VOPProfileMAI_" # P # "_VCD"),
-                                !if(NoDstOverlap, null_frag, VgprMAIFrag<node>)>,
+                                !if(!or(NoDstOverlap, !eq(node, null_frag)), null_frag, VgprMAIFrag<node>)>,
                         MFMATable<0, NAME # "_vgprcd_e64">;
     }
 
     if NoDstOverlap then {
       let Constraints = !if(NoDstOverlap, "$vdst = $src2", ""),
           isConvertibleToThreeAddress = NoDstOverlap,
           Mnemonic = OpName in {
-        def "_mac_e64" : MAIInst<OpName # "_mac", !cast<VOPProfileMAI>("VOPProfileMAI_" # P), AgprMAIFrag<node>>,
+        def "_mac_e64" : MAIInst<OpName # "_mac", !cast<VOPProfileMAI>("VOPProfileMAI_" # P),
+                                 !if(!eq(node, null_frag), null_frag, AgprMAIFrag<node>)>,
                          MFMATable<1, NAME # "_e64">;
 
         let SubtargetPredicate = isGFX90APlus in
         def _mac_vgprcd_e64 : MAIInst<OpName # "_mac_vgprcd", !cast<VOPProfileMAI>("VOPProfileMAI_" # P # "_VCD"),
-                                      VgprMAIFrag<node>>,
+                                      !if(!eq(node, null_frag), null_frag, VgprMAIFrag<node>)>,
                               MFMATable<1, NAME # "_vgprcd_e64">;
       }
     }