Rollup merge of #104605 - RalfJung:clf_consts, r=bjorn3

Rollup merge of #104605 - RalfJung:clf_consts, r=bjorn3

deduplicate constant evaluation in cranelift backend

The cranelift backend had two matches on `ConstantKind`, which can be avoided, and used this `eval_for_mir` that nothing else uses... this makes things more consistent with the (better-tested) LLVM backend.

I noticed this because cranelift was the only user of `eval_for_mir`. However `try_eval_for_mir` still has one other user in `eval`... the odd thing is that the interpreter has its own `eval_mir_constant` which seems to duplicate the same functionality and does not use `try_eval_for_mir`. No idea what is happening here.

r? ``@bjorn3``
Cc ``@lcnr``