Fix logical error when detecting lifetime intrinsics.

Don't replace a gep/bitcast with 'undef' because that will form a "free(undef)"
which in turn means "unreachable". What we wanted was a no-op. Instead, analyze
the whole tree and look for all the instructions we need to delete first, then
delete them second, not relying on the use_list to stay consistent.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136752 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstCombine/malloc-free-delete.ll

@@ -35,3 +35,14 @@ define void @test3() {
   call void @llvm.lifetime.end(i64 10, i8* %a)
   ret void
 }
+
+;; This used to crash.
+define void @test4() {
+; CHECK: @test4
+; CHECK-NEXT: ret void
+  %A = call i8* @malloc(i32 16000)
+  %B = bitcast i8* %A to double*
+  %C = bitcast double* %B to i8*
+  call void @free(i8* %C)
+  ret void
+}