Fix false contract assumption encountered on System.Decimal.op_Explicit

[yaakov-h]

I don't think this has any actual effect on the release builds, but it stopped me getting an exception here when debugging.

op_Explicit, op_Implicit, and op_UnaryNegation all have just one argument, so the assumption that all operations for System.Decimal have two or more arguments is incorrect.

[SergeyTeplyakov]

Two questions:

1. Can we add Contract.Assume(args.Count >= 1); or Contract.Assume(args.Count == 1); for case "op_UnaryNegation":
2. Should we use Contract.Assume(args.Count == 2); instead of Contract.Assume(args.Count >= 2);?

I'm not sure about second one, maybe it can break something, but first one seems very reasonable for me.

[yaakov-h]

1. Yes, I missed that one.
2. I also thought it was odd we were assuming >= when it appears that == is always valid. I don't see what it could break or what could break it.

[SergeyTeplyakov]

Could you please update the PR to address the first comment?
I think we can live with without fixing second one.

[yaakov-h]

Sorry about the delay, it's been nighttime here.

Updated.