[SDAG] Fix llvm.modf for ppc_fp128 (attempt two)

Apparently `DAG.getRoot()` can return null, so we need to check that
case. Hopefully fixes: https://lab.llvm.org/buildbot/#/builders/72/builds/8406

Author: MacDue

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2644,8 +2644,10 @@ bool SelectionDAG::expandMultipleResultFPLibCall(
     // optimized out. This prevents an FP stack pop from being emitted for it.
     // Setting the root like this ensures there will be a use of the
     // `CopyFromReg` chain, and ensures the FP pop will be emitted.
+    SDValue OldRoot = getRoot();
     SDValue NewRoot =
-        getNode(ISD::TokenFactor, DL, MVT::Other, getRoot(), CallChain);
+        OldRoot ? getNode(ISD::TokenFactor, DL, MVT::Other, OldRoot, CallChain)
+                : CallChain;
     setRoot(NewRoot);
     // Ensure the new root is reachable from the results.
     Results[0] = getMergeValues({Results[0], NewRoot}, DL);

llvm/test/CodeGen/PowerPC/llvm.modf.ll

@@ -328,3 +328,25 @@ define { ppc_fp128, ppc_fp128 } @test_modf_ppcf128(ppc_fp128 %a) {
   %result = call { ppc_fp128, ppc_fp128 } @llvm.modf.ppcf128(ppc_fp128 %a)
   ret { ppc_fp128, ppc_fp128 } %result
 }
+
+define ppc_fp128 @test_modf_ppcf128_only_use_intergral(ppc_fp128 %a) {
+; CHECK-LABEL: test_modf_ppcf128_only_use_intergral:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    mflr r0
+; CHECK-NEXT:    stdu r1, -48(r1)
+; CHECK-NEXT:    std r0, 64(r1)
+; CHECK-NEXT:    .cfi_def_cfa_offset 48
+; CHECK-NEXT:    .cfi_offset lr, 16
+; CHECK-NEXT:    addi r5, r1, 32
+; CHECK-NEXT:    bl modfl
+; CHECK-NEXT:    nop
+; CHECK-NEXT:    lfd f1, 32(r1)
+; CHECK-NEXT:    lfd f2, 40(r1)
+; CHECK-NEXT:    addi r1, r1, 48
+; CHECK-NEXT:    ld r0, 16(r1)
+; CHECK-NEXT:    mtlr r0
+; CHECK-NEXT:    blr
+  %result = call { ppc_fp128, ppc_fp128 } @llvm.modf.ppcf128(ppc_fp128 %a)
+  %result.1 = extractvalue { ppc_fp128, ppc_fp128 } %result, 1
+  ret ppc_fp128 %result.1
+}