[AMDGPU] Remove unneeded use of !dag. NFC. (#120546)

Author: jayfoad

llvm/lib/Target/AMDGPU/SIRegisterInfo.td

@@ -903,10 +903,10 @@ multiclass SRegClass<int numRegs,
 
     def SReg_ # suffix :
       SIRegisterClass<"AMDGPU", regTypes, 32,
-                    !con(!dag(add, [!cast<RegisterClass>(sgprName)], ["sgpr"]),
-                    !if(hasTTMP,
-                        !dag(add, [!cast<RegisterClass>(ttmpName)], ["ttmp"]),
-                        (add)))> {
+                      !con((add !cast<RegisterClass>(sgprName)),
+                           !if(hasTTMP,
+                               (add !cast<RegisterClass>(ttmpName)),
+                               (add)))> {
       let isAllocatable = 0;
       let BaseClassOrder = !mul(numRegs, 32);
     }