[HWToBTOR2] Fix crashes on initial value corner cases

[TaoBi22]

Currently HWToBTOR2 crashes on both of the included error cases:

1. unwrapImmutableValue throws an assert whenever it's given a value that wasn't defined by an initialOp, so I've added a check for that. While the first error case isn't technically legal anyway as instance ops aren't allowed in HWToBTOR2, the pass currently crashes before it gets to that point.
2. Initial values given as a block argument will throw an error in requireSort as they don't have a queriable width. Pretty unlikely edge case, but can't hurt to have it covered! I've added it to the list of sorts ignored by requireSort, and it'll now get mopped up by the same check as point 1 rather than crashing

[dobios]

LGTM apart for the small nit! Thanks for catching these!