Added assert for unknown ShiftAmount and an early exit condition for FullValue of zeros and all ones

Author: Tony Varghese

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -15760,6 +15760,9 @@ SDValue PPCTargetLowering::combineBVLoadsSpecialValue(SDValue Op,
     FullVal |= ElemAPInt;
   }
 
+  if (FullVal.isZero() || FullVal.isAllOnes())
+    return SDValue();
+
   if (auto UIMOpt = getPatternInfo(FullVal)) {
     const auto &[Uim, ShiftAmount] = *UIMOpt;
     SDLoc Dl(Op);
@@ -15775,6 +15778,8 @@ SDValue PPCTargetLowering::combineBVLoadsSpecialValue(SDValue Op,
       return LxvkqInstr;
     }
 
+    assert(ShiftAmount == 127 && "Unexpected lxvkq shift amount value");
+
     // The right shifted pattern can be constructed using a combination of
     // XXSPLTIB and VSRQ instruction. VSRQ uses the shift amount from the lower
     // 7 bits of byte 15. This can be specified using XXSPLTIB with immediate