Delete a couple dead checks.

Use castAs<> where appropriate. Don't check conditionals which are
always true. Delete a bit of dead code. Reindent a bunch of code which
is no longer guarded by an if statement.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184801 91177308-0d34-0410-b5e6-96231b3b80d8