[SimpleLoopUnswitch] Check if freeze is needed for partial unswitching.

[fhahn]

Summary

We only need to insert a Freeze instruction if any of the conditions
may be poison. Similar checks are already done in the other places
SimpleLoopUnswitch creates Freeze instruction.

[efriedma]

LGTM

llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

if (FullUnswitch) {
     assert(UnswitchedSuccBBs.size() == 1 &&
            "Only one possible unswitched block for a branch!");
     BasicBlock *ClonedPH = ClonedPHs.begin()->second;
     // When doing a partial unswitch, we have to do a bit more work to build up
     // the branch in the split block.
     if (PartiallyInvariant)
       buildPartialInvariantUnswitchConditionalBranch(
           *SplitBB, Invariants, Direction, *ClonedPH, *LoopPH, L, MSSAU);
-    else
-      buildPartialUnswitchConditionalBranch(*SplitBB, Invariants, Direction,
-                                            *ClonedPH, *LoopPH, InsertFreeze);
+    else {
+      buildPartialUnswitchConditionalBranch(
+          *SplitBB, Invariants, Direction, *ClonedPH, *LoopPH,
+          InsertFreeze && any_of(Invariants, [&](Value *C) {
+            return !isGuaranteedNotToBeUndefOrPoison(C, &AC, BI, &DT);
+          }));
+    }
     DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
 
     if (MSSAU) {
       DT.applyUpdates(DTUpdates);
       DTUpdates.clear();
 
       // Perform MSSA cloning updates.
       for (auto &VMap : VMaps)

llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-freeze.ll

 exit:
   ret void
 }
 
 define i32 @test_partial_unswitch_all_conds_guaranteed_non_poison(i1 noundef %c.1, i1 noundef %c.2) {
 ; CHECK-LABEL: @test_partial_unswitch_all_conds_guaranteed_non_poison(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = and i1 [[C_1:%.*]], [[C_2:%.*]]
-; CHECK-NEXT:    [[DOTFR:%.*]] = freeze i1 [[TMP0]]
-; CHECK-NEXT:    br i1 [[DOTFR]], label [[ENTRY_SPLIT:%.*]], label [[ENTRY_SPLIT_US:%.*]]
+; CHECK-NEXT:    br i1 [[TMP0]], label [[ENTRY_SPLIT:%.*]], label [[ENTRY_SPLIT_US:%.*]]
 ; CHECK:       entry.split.us:
 ; CHECK-NEXT:    br label [[LOOP_US:%.*]]
 ; CHECK:       loop.us:
 ; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @a()
 ; CHECK-NEXT:    br label [[EXIT_SPLIT_US:%.*]]
 ; CHECK:       exit.split.us:
 ; CHECK-NEXT:    br label [[EXIT:%.*]]
 ; CHECK:       entry.split: