Fix MIR unpretty on typeck failure

[nagisa]

Fixes #31913

[rust-highfive]

r? @pnkfelix

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[sanxiyn]

There is another mir_map.unwrap() at pretty.rs:843.

[nagisa]

Fixed.

[arielb1]

@bors r+

[bors]

📌 Commit eb69076 has been approved by arielb1

[bors]

⌛ Testing commit eb69076 with merge d300e4f...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt