These checks should be cheap, so there's little reason for them to be
debug-only.

Historically, these errors existed so that the coverage debug code could dump
additional information before reporting a compiler bug. That debug code was
removed by rust-lang#115962, so we can now simplify these methods by making them panic
when they detect a bug.

…r=oli-obk

Also consider TAIT to be uncomputable if the MIR body is tainted

Not totally sure if this is the best solution. We could, alternatively, look at the hir typeck results and try to take a type from there instead of just falling back to type error, inferring `u8` instead of `{type error}`. Not certain it really matters, though.

Happy to iterate on this.

Fixes rust-lang#117413

r? `@oli-obk` cc `@Nadrieril`

… r=oli-obk

Build a better MIR body when errors are encountered

Doesn't really have much of an effect on its own, but it does lead to a less confusing phony MIR body being generated when an error is detected during THIR/MIR/match building. This was quite confusing when I hacked `-Zunpretty=mir` to emit `mir_built` rather than `instance_mir`.

This coincidentually also fixes rust-lang#117413, but not as generally as rust-lang#117416.

cc `@Nadrieril`

coverage: Replace impossible `coverage::Error` with assertions

Historically, these errors existed so that the coverage debug code could dump additional information before reporting a compiler bug. That debug code was removed by rust-lang#115962, so we can now simplify these methods by making them panic immediately when they detect a bug.

…i-obk

Do not ICE on constant evaluation failure in GVN.

Fixes rust-lang#117362