Casting: assert that pointer arguments to isa<> are non-null.

Casting: assert that pointer arguments to isa<> are non-null.

This silences several analyzer warnings within LLVM, and provides a slightly
nicer crash experience when someone calls isa<>, cast<>, or dyn_cast<> with
a null pointer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164439 91177308-0d34-0410-b5e6-96231b3b80d8