Use isa<> instead of checking return value of definition().

definition() is supposed to be used through isa, dyn_cast or cast.
It's better to not call that directly.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219723 91177308-0d34-0410-b5e6-96231b3b80d8
Remove dead code.

Because we use cast<> at the beginning of this function, it will
abort there if a given atom is not a DefinedAtom.

In the switch statement, we checked if a given atom is a DefinedAtom
again by evaluating definition() == Atom::definitionRegular.
This was always true. So we can remove the outer switch statement.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219724 91177308-0d34-0410-b5e6-96231b3b80d8
Use isa<> and cast<> instead of definition().

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219727 91177308-0d34-0410-b5e6-96231b3b80d8