rustc_trans: don't write discriminants for uninhabited variants

[SimonSapin]

Fixes #46519.

Patch as suggested by eddyb: #46519 (comment)

[rust-highfive]

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[eddyb]

@bors r+

[bors]

📌 Commit 92345a1 has been approved by eddyb

[arielb1]

Test please

@bors r-

[SimonSapin]

Added a test that I confirmed failing in Nightly with:

$ rustc +nightly ~/rust3/src/test/run-pass/issue-46519.rs --test -O            
zsh: floating point exception (core dumped)  rustc +nightly ~/rust3/src/test/run-pass/issue-46519.rs --test -O

[eddyb]

@bors r+

[bors]

📌 Commit d4fabb9 has been approved by eddyb

[bors]

⌛ Testing commit d4fabb9 with merge fa46bca197025c8690a37eb63e29df1246aa529a...

[bors]

💔 Test failed - status-travis

[eddyb]

@bors retry

• some spurious Ruby AWS failure

[bors]

⌛ Testing commit d4fabb9 with merge a62910b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: eddyb
Pushing a62910b to master...