add regression tests for stalled_on const vars

closes rust-lang#70180

Afaict this has been fixed sometime after rust-lang#70213

`trait_ref_type_vars` correctly adds const infers and I did not find any remaining `FIXME`s which correspond to this issue.
https://github.com/rust-lang/rust/blob/7c59a81a5fcbaaca311f744cd7c68d99bfbb05d3/src/librustc_trait_selection/traits/fulfill.rs#L555-L557

Added both examples from the issue as regression tests and renamed `trait_ref_type_vars` -> `trait_ref_infer_vars`.

r? @eddyb

fix ICE when debug-printing MIR

Fixes rust-lang#72105

This bug also makes debugging Miri harder as `MIRI_LOG=info` ICEs.

Fix ice-rust-lang#72442

Closes rust-lang#72442
Closes rust-lang#72426
r? @oli-obk

…r=matthewjasper

Perform MIR NRVO even if types don't match

This is the most straightforward way to resolve rust-lang#72428, but it could cause problems in codegen since the type of `_0` may no longer match the return type of the body.

…-query, r=oli-obk

Removed all instances of const_field.

Fixes rust-lang#72264

r? @oli-obk