[Test] Add test that shows how SimplifyCFG may insert redunant Phi

It happens when a block cannot be threaded because of a convergent function.

test/Transforms/SimplifyCFG/convergent.ll

@@ -0,0 +1,44 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -simplifycfg < %s | FileCheck %s
+; RUN: opt -S -passes=simplify-cfg < %s | FileCheck %s
+
+declare void @foo() convergent
+
+; FIXME: We should not be inserting a PR Phi here.
+define i32 @test_01(i32 %a) {
+; CHECK-LABEL: @test_01(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq i32 [[A:%.*]], 0
+; CHECK-NEXT:    br i1 [[COND]], label [[MERGE:%.*]], label [[IF_FALSE:%.*]]
+; CHECK:       if.false:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    [[COND_PR:%.*]] = phi i1 [ [[COND]], [[IF_FALSE]] ], [ true, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br i1 [[COND_PR]], label [[EXIT:%.*]], label [[IF_FALSE_2:%.*]]
+; CHECK:       if.false.2:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i32 [[A]]
+;
+entry:
+  %cond = icmp eq i32 %a, 0
+  br i1 %cond, label %merge, label %if.false
+
+if.false:
+  call void @foo()
+  br label %merge
+
+merge:
+  call void @foo()
+  br i1 %cond, label %exit, label %if.false.2
+
+if.false.2:
+  call void @foo()
+  br label %exit
+
+exit:
+  ret i32 %a
+}