Add two new items to PerformanceTips

1) Turns out we're not great at recognizing redundant checks when one is a != and the other is an ==.  This is a bug, but it's one that matters to frontend authors.

2) Frontends shouldn't use intrinsics unless strictly neccessary.  This has been pretty widely proven by this point and is good to document.

llvm-svn: 235825