[PowerPC][NFC] Simplify vector unpacked instr classes (llvm#160564)

Apply suggestion as per review comment in
https://github.com/llvm/llvm-project/pull/151004/files#r2240893226

Author: lei137

llvm/lib/Target/PowerPC/PPCInstrFuture.td

@@ -45,79 +45,59 @@ multiclass XOForm_RTAB5_L1r<bits<6> opcode, bits<9> xo, dag OOL, dag IOL,
   }
 }
 
-class VXForm_VRTB5<bits<11> xo, bits<5> R, dag OOL, dag IOL, string asmstr,
-                   list<dag> pattern> : I<4, OOL, IOL, asmstr, NoItinerary> {
+class VXForm_VRTB5_Base<bits<11> xo, dag OOL, dag IOL, string asmstr,
+                        list<dag> pattern>
+    : I<4, OOL, IOL, asmstr, NoItinerary> {
   bits<5> VRT;
   bits<5> VRB;
 
   let Pattern = pattern;
 
   let Inst{6...10} = VRT;
-  let Inst{11...15} = R;
   let Inst{16...20} = VRB;
   let Inst{21...31} = xo;
 }
 
+class VXForm_VRTB5<bits<11> xo, bits<5> R, dag OOL, dag IOL, string asmstr,
+                   list<dag> pattern>
+    : VXForm_VRTB5_Base<xo, OOL, IOL, asmstr, pattern> {
+
+  let Inst{11...15} = R;
+}
+
 class VXForm_VRTB5_UIM2<bits<11> xo, bits<3> R, dag OOL, dag IOL, string asmstr,
                         list<dag> pattern>
-    : I<4, OOL, IOL, asmstr, NoItinerary> {
-  bits<5> VRT;
-  bits<5> VRB;
+    : VXForm_VRTB5_Base<xo, OOL, IOL, asmstr, pattern> {
   bits<2> UIM;
 
-  let Pattern = pattern;
-
-  let Inst{6...10} = VRT;
   let Inst{11...13} = R;
   let Inst{14...15} = UIM;
-  let Inst{16...20} = VRB;
-  let Inst{21...31} = xo;
 }
 
 class VXForm_VRTB5_UIM1<bits<11> xo, bits<4> R, dag OOL, dag IOL, string asmstr,
                         list<dag> pattern>
-    : I<4, OOL, IOL, asmstr, NoItinerary> {
-  bits<5> VRT;
-  bits<5> VRB;
+    : VXForm_VRTB5_Base<xo, OOL, IOL, asmstr, pattern> {
   bits<1> UIM;
 
-  let Pattern = pattern;
-
-  let Inst{6...10} = VRT;
   let Inst{11...14} = R;
   let Inst{15} = UIM;
-  let Inst{16...20} = VRB;
-  let Inst{21...31} = xo;
 }
 
 class VXForm_VRTB5_UIM3<bits<11> xo, bits<2> R, dag OOL, dag IOL, string asmstr,
                         list<dag> pattern>
-    : I<4, OOL, IOL, asmstr, NoItinerary> {
-  bits<5> VRT;
-  bits<5> VRB;
+    : VXForm_VRTB5_Base<xo, OOL, IOL, asmstr, pattern> {
   bits<3> UIM;
 
-  let Pattern = pattern;
-
-  let Inst{6...10} = VRT;
   let Inst{11...12} = R;
   let Inst{13...15} = UIM;
-  let Inst{16...20} = VRB;
-  let Inst{21...31} = xo;
 }
 
 class VXForm_VRTAB5<bits<11> xo, dag OOL, dag IOL, string asmstr,
-                    list<dag> pattern> : I<4, OOL, IOL, asmstr, NoItinerary> {
-  bits<5> VRT;
+                    list<dag> pattern>
+    : VXForm_VRTB5_Base<xo, OOL, IOL, asmstr, pattern> {
   bits<5> VRA;
-  bits<5> VRB;
 
-  let Pattern = pattern;
-
-  let Inst{6...10} = VRT;
   let Inst{11...15} = VRA;
-  let Inst{16...20} = VRB;
-  let Inst{21...31} = xo;
 }
 
 class XX3Form_XTBp5_M2<bits<9> xo, dag OOL, dag IOL, string asmstr,