[PowerPC] Fix assert exposed by PR 95931 in LowerBITCAST (#108062)

Hit Assertion failed: Num < NumOperands && "Invalid child # of SDNode!"
Fix by checking opcode and value type before calling getOperand.

(cherry picked from commit 22067a8)

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

@@ -9338,12 +9338,13 @@ SDValue PPCTargetLowering::LowerBITCAST(SDValue Op, SelectionDAG &DAG) const {
   SDLoc dl(Op);
   SDValue Op0 = Op->getOperand(0);
 
+  if (!Subtarget.isPPC64() || (Op0.getOpcode() != ISD::BUILD_PAIR) ||
+      (Op.getValueType() != MVT::f128))
+    return SDValue();
+
   SDValue Lo = Op0.getOperand(0);
   SDValue Hi = Op0.getOperand(1);
-
-  if ((Op.getValueType() != MVT::f128) ||
-      (Op0.getOpcode() != ISD::BUILD_PAIR) || (Lo.getValueType() != MVT::i64) ||
-      (Hi.getValueType() != MVT::i64) || !Subtarget.isPPC64())
+  if ((Lo.getValueType() != MVT::i64) || (Hi.getValueType() != MVT::i64))
     return SDValue();
 
   if (!Subtarget.isLittleEndian())

llvm/test/CodeGen/PowerPC/f128-bitcast.ll

@@ -86,3 +86,25 @@ entry:
   ret i64 %1
 }
 
+define <4 x i32> @truncBitcast(i512 %a) {
+; CHECK-LABEL: truncBitcast:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    mtvsrdd v2, r4, r3
+; CHECK-NEXT:    blr
+;
+; CHECK-BE-LABEL: truncBitcast:
+; CHECK-BE:       # %bb.0: # %entry
+; CHECK-BE-NEXT:    mtvsrdd v2, r9, r10
+; CHECK-BE-NEXT:    blr
+;
+; CHECK-P8-LABEL: truncBitcast:
+; CHECK-P8:       # %bb.0: # %entry
+; CHECK-P8-NEXT:    mtfprd f0, r3
+; CHECK-P8-NEXT:    mtfprd f1, r4
+; CHECK-P8-NEXT:    xxmrghd v2, vs1, vs0
+; CHECK-P8-NEXT:    blr
+entry:
+  %0 = trunc i512 %a to i128
+  %1 = bitcast i128 %0 to <4 x i32>
+  ret <4 x i32> %1
+}