Subzero: Use "auto" per (unwritten) auto coding style.

auto *Foo = llvm::cast<Foo>(...)
auto *Foo = llvm::dyn_cast<Foo>(...)
auto *Foo = llvm::dyn_cast_or_null<Foo>(...)
auto *Foo = Foo::create(...)

Some instances may have been missed due to line breaks or "const" mismatches.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1516753008 .