Auto merge of #144389 - scottmcm:no-more-mir-cast-assume, r=davidtwco

MIR-build: No longer emit assumes in enum-as casting

This just uses the `valid_range` from the backend, so it's duplicating the range metadata that now we include on parameters and loads, and thus no longer seems to be useful -- notably there's no codegen test failures from removing it.

(Because it's using data from the same source as the backend annotations, it doesn't do anything to mitigate things like rust-lang/rust#144388 where the range in the layout is more permissive than the actual possible discriminants.  A variant of this that actually checked the discriminants more specifically might be useful, so could potentially be added in future, but I don't think the *current* checks are actually providing value.)

r? mir

Randomly turns out that this
Fixes rust-lang/rust#121097

Author: bors