[SimplifyCFG] Remove one-use limitation in FoldCondBranchOnPHI()

BlockIsSimpleEnoughToThreadThrough() already checks that the phi
(and all other instructions) are not used outside the block, so
this one-use check is not necessary for legality. I also don't
see any reason why it would be necessary for profitability (in
fact, those extra uses will be replaced with constants, which
should be generally profitable).

Author: nikic

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2984,9 +2984,7 @@ static Optional<bool> FoldCondBranchOnPHIImpl(BranchInst *BI,
                                               AssumptionCache *AC) {
   BasicBlock *BB = BI->getParent();
   PHINode *PN = dyn_cast<PHINode>(BI->getCondition());
-  // NOTE: we currently cannot transform this case if the PHI node is used
-  // outside of the block.
-  if (!PN || PN->getParent() != BB || !PN->hasOneUse())
+  if (!PN || PN->getParent() != BB)
     return false;
 
   // Degenerate case of a single entry PHI.
@@ -2996,6 +2994,8 @@ static Optional<bool> FoldCondBranchOnPHIImpl(BranchInst *BI,
   }
 
   // Now we know that this block has multiple preds and two succs.
+  // Check that the block is small enough and values defined in the block are
+  // not used outside of it.
   if (!BlockIsSimpleEnoughToThreadThrough(BB))
     return false;
 

llvm/test/Transforms/SimplifyCFG/jump-threading.ll

@@ -50,18 +50,12 @@ define void @test_phi_extra_use(i1 %c) {
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
 ; CHECK:       if:
 ; CHECK-NEXT:    call void @foo()
-; CHECK-NEXT:    br label [[JOIN:%.*]]
-; CHECK:       else:
-; CHECK-NEXT:    call void @bar()
-; CHECK-NEXT:    br label [[JOIN]]
-; CHECK:       join:
-; CHECK-NEXT:    [[C2:%.*]] = phi i1 [ true, [[IF]] ], [ false, [[ELSE]] ]
-; CHECK-NEXT:    call void @use.i1(i1 [[C2]])
-; CHECK-NEXT:    br i1 [[C2]], label [[IF2:%.*]], label [[ELSE2:%.*]]
-; CHECK:       if2:
+; CHECK-NEXT:    call void @use.i1(i1 true)
 ; CHECK-NEXT:    call void @foo()
 ; CHECK-NEXT:    br label [[JOIN2:%.*]]
-; CHECK:       else2:
+; CHECK:       else:
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    call void @use.i1(i1 false)
 ; CHECK-NEXT:    call void @bar()
 ; CHECK-NEXT:    br label [[JOIN2]]
 ; CHECK:       join2: