llvm-prefer-isa-or-dyn-cast-in-conditionals

Looks at conditionals and finds and replaces cases of cast<>, which will assert rather than return a null pointer, and dyn_cast<> where the return value is not captured. Additionally, finds and replaces cases that match the pattern var && isa<X>(var), where var is evaluated twice.

// Finds these:
if (auto x = cast<X>(y)) {}
// is replaced by:
if (auto x = dyn_cast<X>(y)) {}

if (cast<X>(y)) {}
// is replaced by:
if (isa<X>(y)) {}

if (dyn_cast<X>(y)) {}
// is replaced by:
if (isa<X>(y)) {}

if (var && isa<T>(var)) {}
// is replaced by:
if (isa_and_nonnull<T>(var.foo())) {}

// Other cases are ignored, e.g.:
if (auto f = cast<Z>(y)->foo()) {}
if (cast<Z>(y)->foo()) {}
if (X.cast(y)) {}