Handle simulator builtins in dag unroller (#1020)
Handle simulator builtins in dag unroller (#1020)

This commit fixes an issue in the dag unroller when it encounters
simulator builtin instructions like snapshot(). It doesn't know what
those are so it assumes it's a gate. But since it's not a gate when it
tries to look that up it fails on a KeyError. This fixes that by having
a list of simulator builtins to check against and treats those like
other builtins (barrier, reset, etc).

Fixes #1019
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py