tblgen: Mechanically move dynamic_cast<> to dyn_cast<>.

Some of these dyn_cast<>'s would be better phrased as isa<> or cast<>.
That will happen in a future patch.

There are also two dyn_cast_or_null<>'s slipped in instead of
dyn_cast<>'s, since they were causing crashes with just dyn_cast<>.

llvm-svn: 165646