yksmp::Location::Constant should be limited to trace_builder

[ltratt]

We can (mostly as a result of optimisations + side traces) end up with JIT IR like this (a real example):

%50: i1 = load_ti Constant(1)

I thought I could get trace_builder to convert this straight away to:

%50: i1 = Const(1)

but if I do that, things go wrong. I guess this is because "tilocs" are a fixed number of things, and you can't intersperse Inst::Const with them. At least in my particular case I can hack around this in the optimiser, but that might only be a short-term fix.