DebugInfo: Assert dbg.declare/value insts are valid

DebugInfo: Assert dbg.declare/value insts are valid

Remove early returns for when `getVariable()` is null, and just assert
that it never happens.  The Verifier already confirms that there's a
valid variable on these intrinsics, so we should assume the debug info
isn't broken.  I also updated a check for a `!dbg` attachment, which the
Verifier similarly guarantees.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235400 91177308-0d34-0410-b5e6-96231b3b80d8