Rollup merge of #67800 - Aaron1011:fix/mir-generic-instance, r=oli-obk

Rollup merge of #67800 - Aaron1011:fix/mir-generic-instance, r=oli-obk

Fix ICE involving calling `Instance.ty` during const evaluation

Fixes #67639

`Instance.ty` assumes that we are in a fully monomorphic context (e.g.
codegen), and can therefore use an empty `ParamEnv` when performing
normalization. Howver, the MIR constant evaluator code ends up calling
`Instance.ty` as a result of us attemptign to 'speculatively'
const-evaluate generic functions during const propagation.

As a result,
we may end up with projections involving type parameters
(e.g. <T as MyTrait>::Bar>) in the type we are trying to normalize.
Normalization expects us to have proper predicates in the `ParamEnv` for
such projections, and will ICE if we don't.

This commit adds a new method `Instance.ty_env`, which takes a
`ParamEnv` for use during normalization. The MIR const-evaluator code is
changed to use this method, passing in the proper `ParamEnv` for the
context at hand.