SimplifyCFG: fold unconditional branch to its predecessor if profitable.

This patch extends FoldBranchToCommonDest to fold unconditional branches.
For unconditional branches, we fold them if it is easy to update the phi nodes 
in the common successors.

rdar://10554090


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158392 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/SimplifyCFG/branch-fold.ll

@@ -17,3 +17,36 @@ b:
 c:
         ret void
 }
+
+; rdar://10554090
+define zeroext i1 @test2(i64 %i0, i64 %i1) nounwind uwtable readonly ssp {
+entry:
+; CHECK: test2
+; CHECK: br i1
+  %and.i.i = and i64 %i0, 281474976710655
+  %and.i11.i = and i64 %i1, 281474976710655
+  %or.cond = icmp eq i64 %and.i.i, %and.i11.i
+  br i1 %or.cond, label %c, label %a
+
+a:
+; CHECK: br
+  %shr.i4.i = lshr i64 %i0, 48
+  %and.i5.i = and i64 %shr.i4.i, 32767
+  %shr.i.i = lshr i64 %i1, 48
+  %and.i2.i = and i64 %shr.i.i, 32767
+  %cmp9.i = icmp ult i64 %and.i5.i, %and.i2.i
+  br i1 %cmp9.i, label %c, label %b
+
+b:
+; CHECK-NOT: br
+  %shr.i13.i9 = lshr i64 %i1, 48
+  %and.i14.i10 = and i64 %shr.i13.i9, 32767
+  %shr.i.i11 = lshr i64 %i0, 48
+  %and.i11.i12 = and i64 %shr.i.i11, 32767
+  %phitmp = icmp uge i64 %and.i14.i10, %and.i11.i12
+  br label %c
+
+c:
+  %o2 = phi i1 [ false, %a ], [ %phitmp, %b ], [ false, %entry ]
+  ret i1 %o2
+}