[GuardWidening] Fix a nasty cast bug in c2eccc67

c2eccc67 introduced a call to etHasNoUnsignedWrap which implicitly assumes that Inst is a OverflowingBinaryOperator.  This is frequently untrue, but was not caught because cast<Ty>(X) has been broken, see https://discourse.llvm.org/t/cast-x-is-broken-implications-and-proposal-to-address/63033 for context.

I considered reverting this, but since doing so re-introduces a nasty miscompile of its own, I decided to fix forward instead.

I'll note that this is a particularly nasty form of the cast<Ty>(X) issue.  Because the cast was succeeding unexpected, we were writing data to instructions which weren't OBOs.  This could result in near arbitrary data or memory corruption.  I'm a bit shocked that the sanitizers didn't find this TBH.