Teach jump threading some more simple tricks:

1) have it fold "br undef", which does occur with
   surprising frequency as jump threading iterates.
2) teach j-t to delete dead blocks.  This removes the successor
   edges, reducing the in-edges of other blocks, allowing 
   recursive simplification.
3) Fold things like:
     br COND, BBX, BBY
  BBX:
     br COND, BBZ, BBW

   which also happens because jump threading iterates.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60470 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/JumpThreading/basic.ll

@@ -29,3 +29,25 @@ T2:
 F2:
 	ret i32 %B
 }
+
+
+;; cond is known false on Entry -> F1 edge!
+define i32 @test2(i1 %cond) {
+Entry:
+	br i1 %cond, label %T1, label %F1
+
+T1:
+	%v1 = call i32 @f1()
+	br label %Merge
+
+F1:
+	br i1 %cond, label %Merge, label %F2
+
+Merge:
+	%B = phi i32 [47, %T1], [192, %F1]
+	ret i32 %B
+
+F2:
+	call void @f3()
+	ret i32 12
+}